[GIT PULL] execve updates for v6.13-rc1

[GIT PULL] execve updates for v6.13-rc1

[Kees Cook]

Hi Linus,

Please pull these execve updates for v6.13-rc1.

Thanks!

-Kees

The following changes since commit 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b:

  Linux 6.12-rc2 (2024-10-06 15:32:27 -0700)

are available in the Git repository at:

  https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git tags/execve-v6.13-rc1

for you to fetch changes up to 8c9870077aac4b141decfb42431415bb32d1fedf:

  exec: remove legacy custom binfmt modules autoloading (2024-11-16 20:54:03 -0800)

----------------------------------------------------------------
execve updates for v6.13-rc1

- binfmt: Fix comment typos (Christophe JAILLET)

- exec: Use argv[0] for "comm" with AT_EMPTY_PATH (Tycho Andersen,
  Dan Carpenter, Nir Lichtman)

- exec: remove legacy custom binfmt modules autoloading (Nir Lichtman)

- coredump: Do not lock when copying "comm"

- MAINTAINERS: add auxvec.h and set myself as maintainer

----------------------------------------------------------------
Christophe JAILLET (1):
      fs: binfmt: Fix a typo

Dan Carpenter (1):
      exec: Fix a NULL vs IS_ERR() test in bprm_add_fixup_comm()

Kees Cook (4):
      coredump: Do not lock during 'comm' reporting
      MAINTAINERS: exec: Add auxvec.h UAPI
      MAINTAINERS: exec: Mark Kees as maintainer
      exec: NULL out bprm->argv0 when it is an ERR_PTR

Nir Lichtman (1):
      exec: remove legacy custom binfmt modules autoloading

Tycho Andersen (2):
      exec: fix up /proc/pid/comm in the execveat(AT_EMPTY_PATH) case
      selftests/exec: add a test for execveat()'s comm

nir@lichtman.org (1):
      exec: move warning of null argv to be next to the relevant code

 MAINTAINERS                             |  3 +-
 fs/binfmt_misc.c                        |  2 +-
 fs/exec.c                               | 62 ++++++++++++++++++--------
 include/linux/binfmts.h                 |  1 +
 include/linux/coredump.h                |  4 +-
 tools/testing/selftests/exec/execveat.c | 77 +++++++++++++++++++++++++++++++--
 6 files changed, 123 insertions(+), 26 deletions(-)

-- 
Kees Cook

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Tue, 19 Nov 2024 at 09:00, Kees Cook <kees@kernel.org> wrote:
>
> - exec: Use argv[0] for "comm" with AT_EMPTY_PATH (Tycho Andersen,
>   Dan Carpenter, Nir Lichtman)

Ugh. I *really* despise this one.

People: we *have* a filename. It's right there in the dentry. Which is
right there as bprm->file->f_dentry.dentry.

And that's actually going to match the actual execcutable, unlike, for
example, argv[0], which can be filled in with random data.

*AND* we don't need any silly and expensive get_user_arg_ptr() and
strndup_user() copy for it, which does that user access twice.

And no, we shouldn't fall back to the horrid thing that bprm->fdpath
does either. That's the thing that you apparently thought was too ugly
to use, and literally the *only* use of it was for this case.

The reason that code existed at all was to generate a filename, and
because we didn't use to have access to the 'bprm->file' back in the
days.

But that was changed by commit 978ffcbf00d8 ("execve: open the
executable file before doing anything else").

So I really really think that what this code *should* have done is

 - get rid of fdpath that you made pointless by not using it for 'comm[]'

 - teach the code to use the dentry name instead

because this horrible hack is too broken to live.

             Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Wed, 20 Nov 2024 at 14:32, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> People: we *have* a filename. It's right there in the dentry. Which is
> right there as bprm->file->f_dentry.dentry.

... that should obviously be '...->f_path.dentry'.

             Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Wed, 20 Nov 2024 at 14:33, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, 20 Nov 2024 at 14:32, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > People: we *have* a filename. It's right there in the dentry. Which is
> > right there as bprm->file->f_dentry.dentry.
>
> ... that should obviously be '...->f_path.dentry'.

One thing to look out for is that dentry->name can be switched around
by renames etc

So you probably want to do something like

        const char *name = smp_load_acquire(&dentry->d_name.name);

under the RCU read lock before then copying it with strscpy(). It
should always be NULL-terminated.

If you want to be extra careful, you might surround it with a

        read_seqbegin_or_lock(&rename_lock, &seq);

        ..

        if (need_seqretry(&rename_lock, seq)) {
                seq = 1;
                goto restart;
        }
        done_seqretry(&rename_lock, seq);

but I seriously doubt we even care that much. If people want to mess
with the executable filename, they can just use links or whatever, so
this is a "politeness" thing rather than anything else.

(And yes, our d_path() code tries to be extra smart and also uses
'name->len' to avoid having to search for the end of the string etc,
but that then causes huge complexities with ->len and ->name not
matching, so it has to compensate for that with being extra careful.
So don't do that)

                     Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Al Viro]

On Wed, Nov 20, 2024 at 02:50:39PM -0800, Linus Torvalds wrote:
> So you probably want to do something like
> 
>         const char *name = smp_load_acquire(&dentry->d_name.name);
> 
> under the RCU read lock before then copying it with strscpy(). It
> should always be NULL-terminated.
> 
> If you want to be extra careful, you might surround it with a
> 
>         read_seqbegin_or_lock(&rename_lock, &seq);

What for?

	char name[something];

	sprintf(name, "%*pD", sizeof(name) - 1, file);

and be done with that...

Re: [GIT PULL] execve updates for v6.13-rc1

[Al Viro]

On Thu, Nov 21, 2024 at 02:36:19AM +0000, Al Viro wrote:
> On Wed, Nov 20, 2024 at 02:50:39PM -0800, Linus Torvalds wrote:
> > So you probably want to do something like
> > 
> >         const char *name = smp_load_acquire(&dentry->d_name.name);
> > 
> > under the RCU read lock before then copying it with strscpy(). It
> > should always be NULL-terminated.
> > 
> > If you want to be extra careful, you might surround it with a
> > 
> >         read_seqbegin_or_lock(&rename_lock, &seq);
> 
> What for?
> 
> 	char name[something];
> 
> 	sprintf(name, "%*pD", sizeof(name) - 1, file);
> 
> and be done with that...

... or

	struct name_snapshot n;

	take_dentry_name_snapshot(&n, file->f_path.dentry);

	do_something(n.name.name);

	release_dentry_name_snapshot(&n);

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Wed, 20 Nov 2024 at 18:36, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > If you want to be extra careful, you might surround it with a
> >
> >         read_seqbegin_or_lock(&rename_lock, &seq);
>
> What for?

Yeah, probably no good reason.

>         char name[something];
>
>         sprintf(name, "%*pD", sizeof(name) - 1, file);
>
> and be done with that...

I think you might as well just do the much cheaper

        rcu_read_lock();
        strscpy_pad(tsk->comm, sizeof(tsk->comm),
                smp_read_acquire(&dentry->d_name.name);
        rcu_read_unlock();

and be done with it. Using snprintf() on tsk->comm[] violates all the
rules we have for it, and while our rules aren't great, *that* one we
shouldn't do.

                 Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Eric W. Biederman]

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Tue, 19 Nov 2024 at 09:00, Kees Cook <kees@kernel.org> wrote:
>>
>> - exec: Use argv[0] for "comm" with AT_EMPTY_PATH (Tycho Andersen,
>>   Dan Carpenter, Nir Lichtman)
>
> Ugh. I *really* despise this one.
>
> People: we *have* a filename. It's right there in the dentry. Which is
> right there as bprm->file->f_dentry.dentry.

__set_task_comm cannot be called with bprm->file->f_dentry
unconditionally.  That will break userspace.  On at least debian using
/etc/alternatives there are a lot of symlinks to executables.

For example if I run "vi", two symlinks are followed:
/usr/bin/vi -> /etc/alternatives/vi
/etc/alternatives/vi -> usr/bin/vim.nox

Seeing "vim.nox" in ps instead "vi" would be weird and break tooks like
psgrep.

https://lore.kernel.org/all/Zv1OayMEmLP2kjhj@kawka3.in.waw.pl/

The reason bprm->file->f_dentry.dentry was abandoned were concerns
about breaking userspace.


Limited to just the binaries that systemd wants to call.  I don't know
if symlinks to binaries matters.  At a quick skim I couldn't see
anything but *shrug*.


> And that's actually going to match the actual execcutable, unlike, for
> example, argv[0], which can be filled in with random data.
>
>
> *AND* we don't need any silly and expensive get_user_arg_ptr() and
> strndup_user() copy for it, which does that user access twice.
>
> And no, we shouldn't fall back to the horrid thing that bprm->fdpath
> does either. That's the thing that you apparently thought was too ugly
> to use, and literally the *only* use of it was for this case.

bprm->fdpath is for passing the pathname of the script to an interpreter
like /bin/sh.  That task->comm wind up being filled from it, is a bit
of a surprise.

> The reason that code existed at all was to generate a filename, and
> because we didn't use to have access to the 'bprm->file' back in the
> days.
>
> But that was changed by commit 978ffcbf00d8 ("execve: open the
> executable file before doing anything else").

That should work for binfmt_script.  I don't know which solution
is less racy, and less prone to problems in the binfmt_script case.
No one has complained so I have been assuming that case works just
fine.

> So I really really think that what this code *should* have done is
>
>  - get rid of fdpath that you made pointless by not using it for 'comm[]'

Again binfmt_script still uses it.

I can assure you there was no thought at all given to task->comm when
the contents of bprm->fdpath were chosen.  Which is why there is
a problem now.

task->comm is for people.  bprm->fdpath is for programs.

>  - teach the code to use the dentry name instead

Which is fine for task->comm, when the pathname is NULL. Assuming it
actually makes things usable for userspace.

That doesn't work for bprm->fdpath with becomes bprm->interp.  That
would require using d_path.

Eric

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Wed, 20 Nov 2024 at 16:55, Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> __set_task_comm cannot be called with bprm->file->f_dentry
> unconditionally.

No, no. Only for the "no path" case.

> The reason bprm->file->f_dentry.dentry was abandoned were concerns
> about breaking userspace.

There's no way it can break user space considering that right now
comm[] ends up being just garbage.

And I do *not* want to replace one garbage with a new one.

> >  - get rid of fdpath that you made pointless by not using it for 'comm[]'
>
> Again binfmt_script still uses it.

Ahh, yeah, we can't just get rid of it.

But we're *not* adding a new complete garbage "copy random stuff badly
from user space" thing.

             Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Kees Cook]

On November 20, 2024 6:23:55 PM PST, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>But we're *not* adding a new complete garbage "copy random stuff badly
>from user space" thing.

I'll drop this and resend the PR.

I think using the dentry should work, as you say. Though with a more paranoid hat on, I wonder if it could be an info leak if the original filename isn't "visible" in the context it's running in; containers do weird stuff. (But I'm hard-pressed to imagine how the filename would actually be a useful "secret".)

-Kees

-- 
Kees Cook

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Wed, 20 Nov 2024 at 18:23, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> > Again binfmt_script still uses it.
>
> Ahh, yeah, we can't just get rid of it.

Actually, that reminds me: we've had issues with this horrible fdpath
hack before due to close-on-exec, and that's why we have
BINPRM_FLAGS_PATH_INACCESSIBLE.

And that's independent of the whole "/proc isn't always mounted", so
that the /dev/fd/%d/.. paths don't work at all.

It would probably have been much nicer if we just put the real path of
the dentry originally in ->fdpath, but I suspect it's too late to fix
now: it would _mostly_ be a more reliable and meaningful path, and it
would fix the close-on-exec situation, but I would not be surprised if
we have some horrible  user that really depends on the 'fd' being the
only way to actually access it (either due to permission issues, or
because of it having been actively unlinked).

              Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Wed, Nov 20, 2024 at 06:23:55PM -0800, Linus Torvalds wrote:
> On Wed, 20 Nov 2024 at 16:55, Eric W. Biederman <ebiederm@xmission.com> wrote:
> >
> > __set_task_comm cannot be called with bprm->file->f_dentry
> > unconditionally.
> 
> No, no. Only for the "no path" case.
> 
> > The reason bprm->file->f_dentry.dentry was abandoned were concerns
> > about breaking userspace.
> 
> There's no way it can break user space considering that right now
> comm[] ends up being just garbage.

It'll "break userspace" in the sense the the resulting program name
visible in /proc/self/{comm,stat,status} would be different than the
expected value. Currently userspace is not using fexecve because this
string is "just garbage". We'd very much like to start using fexecve,
but we cannot do this (in the general case) if that'll result in a
changed program name. If we change the value from the current
(garbage) value to something that doesn't provide identical behaviour
between execve and fexecve, fexecve will unused.

As Eric wrote, there are various programs which are symlinked.
/etc/alternatives is one group, but we also have "multicall binaries"
which present different behaviour depending on the name.
Some of those use argv[0], but other may use comm.
We really need the name that the user called the program as,
not the name after symlink chasing.

Even if we end up copying a string from userspace unnecessarilly,
does this matter? execve is a heavyweight operation and copying a
a dozen bytes extra hardly matters.

Zbyszek

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Thu, 21 Nov 2024 at 09:22, Zbigniew Jędrzejewski-Szmek
<zbyszek@in.waw.pl> wrote:
>
> It'll "break userspace" in the sense the the resulting program name
> visible in /proc/self/{comm,stat,status} would be different than the
> expected value. Currently userspace is not using fexecve because this
> string is "just garbage". We'd very much like to start using fexecve,
> but we cannot do this (in the general case) if that'll result in a
> changed program name. If we change the value from the current
> (garbage) value to something that doesn't provide identical behaviour
> between execve and fexecve, fexecve will unused.

Well, then you had better not use fexecve(), because that "identical
behavior" is fundamentally impossible.

The thing is, "argv[0]" can - and will be - complete garbage. Yes,
it's *often* the same as the filename, but there is actually zero
guarantee of that. It can be any random thing - it's literally just a
user space argument.

And the dentry name *will* be the name of the underlying executable.
Again, it is *often* the same as the filename, but symlinks have
already been brought up as an example when it isn't.

See? There is no single solution, but at least the dentry name is a
*reliable* thing, not a random garbage thing passed in by user space.

End result: if you don't like it, don't use fexecve(). It really is that simple.

               Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Thu, 21 Nov 2024 at 09:28, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> See? There is no single solution, but at least the dentry name is a
> *reliable* thing, not a random garbage thing passed in by user space.

Note that the reason I want 'comm[]' to be something actually
*reliable* is that that is what the kernel actually uses for various
error messages etc.

User space tools like 'ps' already do the "dig around in user space to
look for argv[]" thing, and tools like "ps" do *not* actually use
comm[] at all from normal user space programs.

For example, why do you think "ps 3545" says

   3545 ?        Ssl    0:00 /usr/libexec/gnome-shell-calendar-server

but when I do "cat /proc/3545/stat" I see

    3545 (gnome-shell-cal) S 3129 ...

in the output?

That's exactly because comm[] has that "gnome-shell-cal" (limited to
16 characters with the NUL due to TASK_COMM_LEN), but 'ps' will go
digging into user space argv[0] by looking at /proc/*/cmdline which
gets you much more than just the name of the executable.

And I do *not* want anybody to think that "comm[]" should act IN ANY
WAY identically to /proc/*/cmdline. It never has, and it never will.
Exactly because the two are completely different things, for different
uses.

And yes, people have historically actively messed with the argv[0]
thing, and actively tried to hide what the actual executable was.

I am not AT ALL interested in letting people play those kinds of games
with "comm[]".

There's a very real reason I rejected that original execve() change.
It was stupid and wrong.

                  Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Thu, Nov 21, 2024 at 09:28:09AM -0800, Linus Torvalds wrote:
> On Thu, 21 Nov 2024 at 09:22, Zbigniew Jędrzejewski-Szmek
> <zbyszek@in.waw.pl> wrote:
> >
> > It'll "break userspace" in the sense the the resulting program name
> > visible in /proc/self/{comm,stat,status} would be different than the
> > expected value. Currently userspace is not using fexecve because this
> > string is "just garbage". We'd very much like to start using fexecve,
> > but we cannot do this (in the general case) if that'll result in a
> > changed program name. If we change the value from the current
> > (garbage) value to something that doesn't provide identical behaviour
> > between execve and fexecve, fexecve will unused.
> 
> Well, then you had better not use fexecve(), because that "identical
> behavior" is fundamentally impossible.

Identical — as far as the callee is concerned.
Basically, we'd like to switch the execve() that we use in systemd
to start everything with fexecve(), but this should be invisible to
both the programs that are started and users who call ps/pgrep/….

> The thing is, "argv[0]" can - and will be - complete garbage. Yes,
> it's *often* the same as the filename, but there is actually zero
> guarantee of that. It can be any random thing - it's literally just a
> user space argument.

Eh, no. I think you're trying to say that argv[0] is user-controlled.
Sure, this is a feature. Systemd even exposes this as
  ExecStart=@program argv0 argv1 argv2…
It can be overridden, but it's not "garbage".

> And the dentry name *will* be the name of the underlying executable.
> Again, it is *often* the same as the filename, but symlinks have
> already been brought up as an example when it isn't.
Exactly. This is the crux of the problem. We think that fd-based
syscalls are great, and would like to use fexecve as a drop-in
replacement, but currently can't. I would love to tell the rest
of the userspace to stop ever looking at COMM, but, as you very
well know, once an API is made public and widely used, it's very
hard to redefine it.

> See? There is no single solution, but at least the dentry name is a
> *reliable* thing, not a random garbage thing passed in by user space.

Reliable – yes. Useful – no.

Zbyszek

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Thu, 21 Nov 2024 at 10:00, Zbigniew Jędrzejewski-Szmek
<zbyszek@in.waw.pl> wrote:
>
> Identical — as far as the callee is concerned.
> Basically, we'd like to switch the execve() that we use in systemd
> to start everything with fexecve(), but this should be invisible to
> both the programs that are started and users who call ps/pgrep/….

I'm not discussing this. If you cannot understand the difference
between comm[] and argv[0], this discussion is entirely pointless.

I'd suggest you just not use fexecve().

            Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Thu, Nov 21, 2024 at 10:02:03AM -0800, Linus Torvalds wrote:
> On Thu, 21 Nov 2024 at 10:00, Zbigniew Jędrzejewski-Szmek
> <zbyszek@in.waw.pl> wrote:
> >
> > Identical — as far as the callee is concerned.
> > Basically, we'd like to switch the execve() that we use in systemd
> > to start everything with fexecve(), but this should be invisible to
> > both the programs that are started and users who call ps/pgrep/….
> 
> I'm not discussing this. If you cannot understand the difference
> between comm[] and argv[0], this discussion is entirely pointless.

You brought up argv to say that it's "garbage". I was replying to that
part of your message.

With execve, the initial values of comm and argv are under full
control of the caller, e.g. comm can be set as
'ln -s /bin/sleep /tmp/whatever && /tmp/whatever'.
So comm doesn't have to match the "actual executable name" and
if one is "garbage" then so is the other one.

I very much understand the difference between comm and argv[0].
Once again: the goal is to be able to use fexecve in a way that
doesn't cause a visible change to the called programs. Comm was
previously set to the basename of the path, and if it's set based on
argv[0] with fexecve, we get the behaviour that we want (*) and the
rest of userspace is happy. If we set it to f_path.dentry, userspace
is unhappy.

(*) The exception is when a program is called with an argv[0] that
has some special value. But it's such a niche case that nobody
cares about it. There were some cups printer plugins which passed
something important via argv[0], but that was at least a decade
ago. I'm not aware of anybody trying to do that. In the worst case,
we can fall back to execve for those cases if absolutely necessary.

> I'd suggest you just not use fexecve().

Pffff. That's not helpful. The patch to use fexecve/execveat
in systemd was merged in 2020, but we can't make this the default
behaviour because of the comm blocker.

Zbyszek

Re: [GIT PULL] execve updates for v6.13-rc1

[Kees Cook]

On Thu, Nov 21, 2024 at 10:02:03AM -0800, Linus Torvalds wrote:
> On Thu, 21 Nov 2024 at 10:00, Zbigniew Jędrzejewski-Szmek
> <zbyszek@in.waw.pl> wrote:
> >
> > Identical — as far as the callee is concerned.
> > Basically, we'd like to switch the execve() that we use in systemd
> > to start everything with fexecve(), but this should be invisible to
> > both the programs that are started and users who call ps/pgrep/….
> 
> I'm not discussing this. If you cannot understand the difference
> between comm[] and argv[0], this discussion is entirely pointless.
> 
> I'd suggest you just not use fexecve().

I think you're talking past each other. Here's my thought process:

To Linus's point, comm[] is "garbage" in that a process can change it to
anything it wants (i.e. PR_SET_NAME). I think everyone understands this,
but that's not what what I see as the issue.

What commp[] does have, however, is a "default" (starting) value set by
execve(), which is that of the basename of the "pathname" argument of
the syscall (yes, not argv[0], but see below).

Most process list analysis tools (i.e. "ps") use /proc/*/comm for the
short name view of a process. Most process launchers (i.e. shells,
systemd), tend to use basename(pathname) as argv[0] when running
programs. Again, I think everyone understands these things too, but I
think it's worth calling out that while comm[] and argv[0] are obviously
different things, in most cases they start out with identical values
after execve().

The problem is fexecve(), where the pathname is lost since it was,
obviously, only passed to open(). Right now, we re-use bprm->fdpath since
it was already there for scripts, but that unhelpfully just shoves the
fd number into comm[], making "ps" output unreadable. Nobody likes
seeing basename(fdpath) in comm[] today.

I don't think it's unreasonable to want to retain the basename(pathname)
starting value of comm[] when switching from execve() to fexecve().

Using the f_path name is certainly better than basename(fdpath), but it
doesn't really give userspace what they'd like nor expect. Since comm[]
is mutable anyway, why not just copy argv[0] for this case, as that
would give userspace exactly what it was expecting and does no harm?
i.e. yes, comm[] is "garbage", but let's replicate in fexecve() as close
to the default behavior we have from execve() as we can. Why expose
f_path, which doesn't appear in comm[] nor cmdline currently?

The only flip side I can see is that "ps" etc, should just never use comm
at all, and instead use argv[0] from cmdline. That would get the same
behavior as described here, but it is much more expensive in that is takes
the mm lock and has to walk userspace memory. But then we don't need
to change anything on the kernel side and just leave basename(fdpath)
alone. I would note, of course, that cmdline can also be changed by the
process (PR_SET_MM_ARG_START), too, so it is also "garbage". Just more
expensive garbage than comm[].

I still think the original proposal is the most helpful to userspace.

-Kees

-- 
Kees Cook

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Thu, 21 Nov 2024 at 10:50, Kees Cook <kees@kernel.org> wrote:
>
> The only flip side I can see is that "ps" etc, should just never use comm
> at all, and instead use argv[0] from cmdline

Gods people, what are you all on about?

THIS IS WHAT PS ALREADY DOES.

Stop this completely inane discussion. It's literally like you don't
even know what you are talking about.

For user space, comm[] is basically the fallback for when cmdline
fails for some reason (for example, /proc/*/cmdline will be empty for
kworkers, but there are other situations too)

The reason? comm[] has *always* been much too limited for 'ps' output. ALWAYS.

Yes, you can literally *force* ps to not do that (eg "ps -eo comm")
but if you do that, you get the very limited comm[] output that nobody
has ever wanted ps to give exactly because it's so limited.

People who want 'argv[0]' will typically want argv[1] etc too, because
argv[0] simply IS NOT SPECIAL.

And yes, 'top' will give comm[] output because it's so much faster.

I'm done with this discussion that apparently was brought on by people
not knowing what the hell they were doing.

                Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Thu, Nov 21, 2024 at 11:23:46AM -0800, Linus Torvalds wrote:
> On Thu, 21 Nov 2024 at 10:50, Kees Cook <kees@kernel.org> wrote:
> >
> > The only flip side I can see is that "ps" etc, should just never use comm
> > at all, and instead use argv[0] from cmdline
> 
> Gods people, what are you all on about?
> 
> THIS IS WHAT PS ALREADY DOES.

Nope. 'ps' uses comm, for example in 'ps -C' which is documented as
"select by command name" and quite commonly used.

It does not use it in the default listing, because it shows "COMMAND",
incl. args, which obviously must use /proc//cmdline. But nobody said
that it does. Kees only mentioned that "ps uses comm for the short
name view of a process", which is true. _You_ wrote about ps and showed
an example with COMMAND which is … fine, but not really relevant.

If you wanted, you could ask where is comm used by the userspace?
The ones I'm aware of where it's directly visible:
- 'ps -C' or 'ps -o comm'
- pgrep
- top
- htop

Using sourcegraph.com I see mentions in moby, kubernetes, zsh and
ohmyzsh, earlyoom, vmtop, and pages of more obscure stuff. It's also
exposed via libsystemd's sd_bus_creds and journald's _COMM field.
The generic interfaces are the biggest problem. We could probably
update top/htop/pgrep to use argv[0], but we can't fix scripts
that are out there that either use 'ps -o comm' or look in /proc//comm
directly or filter journalctl output by field.

> Stop this completely inane discussion. It's literally like you don't
> even know what you are talking about.
>
> For user space, comm[] is basically the fallback for when cmdline
> fails for some reason (for example, /proc/*/cmdline will be empty for
> kworkers, but there are other situations too)

As mentioned, this is easily disproved by running e.g. top/htop/pgrep.

> The reason? comm[] has *always* been much too limited for 'ps' output. ALWAYS.
> 
> Yes, you can literally *force* ps to not do that (eg "ps -eo comm")
> but if you do that, you get the very limited comm[] output that nobody
> has ever wanted ps to give exactly because it's so limited.
> 
> People who want 'argv[0]' will typically want argv[1] etc too, because
> argv[0] simply IS NOT SPECIAL.
> 
> And yes, 'top' will give comm[] output because it's so much faster.

Exactly.

> I'm done with this discussion that apparently was brought on by people
> not knowing what the hell they were doing.

You keep disagreeing with things that nobody has said.

The point is that comm _is_ used in many places that matter. I'm not
sure what you're trying to say really, since in the second half of
your mail you actually showed an example where this is true.

Zbyszek

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Thu, 21 Nov 2024 at 13:31, Zbigniew Jędrzejewski-Szmek
<zbyszek@in.waw.pl> wrote:
>
> The point is that comm _is_ used in many places that matter. I'm not
> sure what you're trying to say really, since in the second half of
> your mail you actually showed an example where this is true.

Really, let me say this one more time: if you don't like the dentry
name, don't give execveat() a bare file descriptor that has no other
name, and claim that you want to use some completely unrelated third
thing that has nothing to do with it except in your little
dream-world.

The dentry name is not just fine, it's THE TRUTH. It's fundamentally
what you are executing.

It's better than - and fundamentally different from - argv[0], which
is something entirely different and is defined to be available
elsewhere.

If you have done an open() that followed a symlink, the name *behind*
the symlink is the *CORRECT* name. So when you then do an execveat(),
it's literally the thing *behind* the symlink that you are executing.

You are trying to claim that people want argv[0]. No they don't. If
they wanted argv[0], they'd use /proc/*/cmdline, which already give
you that, and more.

And if they don't want argv[0], then they get the name that the
executable was started with. Which is the name *behind* the symlink
that was resolved by the (preceding) 'open()'.

I'm trying to explain to you that comm[] IS NOT, AND HAS NEVER BEEN,
argv[0]. The two have *never* matched up and been the same thing. It's
simply not how execve() works.

I understand that you seem to *think* that it is how execve() works or
should work, but that's your hangup, and has no actual basis in
reality.

I'm telling you that comm[] is not argv[0], and has never been argv[0]. Ever.

I will not take that idiotic patch that tries to change decades of
history, based on what is purely your personal misunderstanding of
what argv[0] is, and does something *stupid* and slow, when something
much more true is right there in the dentry name.

I don't even understand why you care about the symlink thing. You did
an open, you followed the symlink, the file no longer points to the
symlink, trying to claim that it does is simply not *true* in any
shape or form.

Here's a hint for you: do the same open, but never execute it. Then
look in /proc/*/fd and see what that says. Horror of horrors, it
doesn't show the symlink. It shows what it pointed to.

And before you start arguing about that too, let me just say that
we're not changing that *either*.

           Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Thu, Nov 21, 2024 at 01:48:59PM -0800, Linus Torvalds wrote:
> On Thu, 21 Nov 2024 at 13:31, Zbigniew Jędrzejewski-Szmek
> <zbyszek@in.waw.pl> wrote:
> >
> > The point is that comm _is_ used in many places that matter. I'm not
> > sure what you're trying to say really, since in the second half of
> > your mail you actually showed an example where this is true.
> 
> Really, let me say this one more time: if you don't like the dentry
> name, don't give execveat() a bare file descriptor that has no other
> name, and claim that you want to use some completely unrelated third
> thing that has nothing to do with it except in your little
> dream-world.
> 
> The dentry name is not just fine, it's THE TRUTH. It's fundamentally
> what you are executing.

It may be THE TRUTH, but this is not what userspace expects.
A user calls e.g. /usr/bin/wget and they really expect 'pgrep wget'
to return a PID. They don't care that the dentry name is actually
'wget2', even if the exec is realized through fexecve.

> It's better than - and fundamentally different from - argv[0], which
> is something entirely different and is defined to be available
> elsewhere.

Kees explained this really well, please check his mail. Everybody
knows that argv[0] is (or rather can be) different.

What people expect is for /proc//comm to be the basename of the path
that was passed to execve(2). Give me an interface that allows the
caller to use fexecve() instead and set comm. I don't really care how
that interface looks, as long as it's possible to set comm to
mimick previous behaviour.

The patch uses argv[0] because in a great majority of cases in normal
use basename(argv[0]) is the same as comm. So it provides a good
fallback. _Better_ than the dentry name, as far as the userspace is
concerned.

> If you have done an open() that followed a symlink, the name *behind*
> the symlink is the *CORRECT* name. So when you then do an execveat(),
> it's literally the thing *behind* the symlink that you are executing.
> 
> You are trying to claim that people want argv[0]. No they don't. If
> they wanted argv[0], they'd use /proc/*/cmdline, which already give
> you that, and more.

That's not what I said. What was said is that argv[0] can be used
to set comm in a way that _the userspace expects_.

To be clear: the dentry name goes 95% of the way. It certainly
is better than the current "3" or whatever. If we cannot fix the
remaining 5%, we'll need to adjust some userspace and possibly
have some annoyed users. So I'd prefer that the kernel merges a
patch that uses the dentry name, but it'd be better to use
argv[0] which solves the problem 99+% of the way.

> And if they don't want argv[0], then they get the name that the
> executable was started with. Which is the name *behind* the symlink
> that was resolved by the (preceding) 'open()'.
> 
> I'm trying to explain to you that comm[] IS NOT, AND HAS NEVER BEEN,
> argv[0]. The two have *never* matched up and been the same thing. It's
> simply not how execve() works.

Yes, this is exactly what Kees wrote in his mail. Nobody is disputing
that.

> I don't even understand why you care about the symlink thing. You did
> an open, you followed the symlink, the file no longer points to the
> symlink, trying to claim that it does is simply not *true* in any
> shape or form.

I have a generic execution engine (systemd). Userspace uses it to call
arbitrary programs. That userspace expects that comm/pgrep/top/htop/etc/etc/etc
show the "name of the program". The _name by which it was called_.
For decades, this has been the symlink name, not target.

Zbyszek

Re: [GIT PULL] execve updates for v6.13-rc1

[Kees Cook]

On Thu, Nov 21, 2024 at 11:23:46AM -0800, Linus Torvalds wrote:
> I'm done with this discussion that apparently was brought on by people
> not knowing what the hell they were doing.

This is disrespectful. If you're frustrated you can just say so. I'm
certainly frustrated.

> For user space, comm[] is basically the fallback for when cmdline
> fails for some reason (for example, /proc/*/cmdline will be empty for
> kworkers, but there are other situations too)
> 
> The reason? comm[] has *always* been much too limited for 'ps' output. ALWAYS.
> 
> Yes, you can literally *force* ps to not do that (eg "ps -eo comm")
> but if you do that, you get the very limited comm[] output that nobody
> has ever wanted ps to give exactly because it's so limited.

I think I finally figured out why you keep saying this. I think you mean
to imply "ps -e" (or similar), not "ps". Asking for more process details
("ps a", "ps -f", "ps -e", etc) uses cmdline. Without options that turn
on more details, the default is comm. If you run just "ps", it shows comm:

$ strace ps 2>&1 | grep /cmdline | wc -l
0

If you run with detail options it shows cmdline:

$ strace ps a 2>&1 | grep /cmdline | wc -l
1266
$ strace ps -f 2>&1 | grep /cmdline | wc -l
1266

This is procps-ng found on all Debian and Ubuntu systems. AFAICT
procps-ng is used on Fedora too.

Note I'm not saying comm is GOOD or anything. I'm just saying that it
IS regularly visible.

> And yes, 'top' will give comm[] output because it's so much faster.

Exactly. By default, top and ps both show comm, which in the vast
majority of cases is identical to argv[0]. I don't understand why this
is causing such angst -- it's just the observable facts: many things in
userspace expose comm and many also expose cmdline. Having them be
mismatched due to fexecve() is the whole issue.

Nobody blinks at:

    $ ps
        PID TTY          TIME CMD
    4125309 pts/1    00:00:47 bash
    4171960 pts/1    00:00:00 bash
    4171962 pts/1    00:00:00 vim
    4171997 pts/1    00:00:00 ps

vs

    $ ps -f
    UID          PID    PPID  C STIME TTY          TIME CMD
    kees     4125309    3947  0 Jul11 pts/1    00:00:47 -bash
    kees     4171960 4125309  0 13:30 pts/1    00:00:00 -bash
    kees     4171962 4171960  0 13:30 pts/1    00:00:00 vim
    kees     4172004 4125309  0 13:30 pts/1    00:00:00 ps -f

But if fexecve() were used now, "ps" would show:

    $ ps
        PID TTY          TIME CMD
    4125309 pts/1    00:00:47 3
    4171960 pts/1    00:00:00 3
    4171962 pts/1    00:00:00 3
    4171997 pts/1    00:00:00 3

This is obviously horrible.

Using f_path, we'd get close, but symlink destinations (or weird stuff
like "memfd:name-here") are shown:

    $ realpath $(which vim)
    /usr/bin/vim.basic

    $ ps
        PID TTY          TIME CMD
    4125309 pts/1    00:00:47 bash
    4171960 pts/1    00:00:00 bash
    4171962 pts/1    00:00:00 vim.basic
    4171997 pts/1    00:00:00 ps

Using argv[0], we'd get the original output:

    $ ps
        PID TTY          TIME CMD
    4125309 pts/1    00:00:47 bash
    4171960 pts/1    00:00:00 bash
    4171962 pts/1    00:00:00 vim
    4171997 pts/1    00:00:00 ps

IMO, switching to fexecve() shouldn't regress this basic piece of
information.

Now, I think we have three choices:

1) Leave it as-is. (comm is useless)

2) Use argv[0]. (comm matches what would show with execve() in most cases)

3) Use f_path (comm exposes f_path dentry name, which matches
   basename(readlink(/proc/*/exe)), but doesn't always match what execve()
   would show).

I think everyone agrees "1" should go away.

So it comes down to trying to stay looking more like execve()'s comm, or
more like /proc/*/exe's value.

Since comm is mutable anyway, I feel like the "friendlier" default for
userspace would be option 2.

If you still conclude differently, I guess the discussion is over, and
we go with 3:

diff --git a/fs/exec.c b/fs/exec.c
index e0435b31a811..8688bbbaf4af 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1347,7 +1347,21 @@ int begin_new_exec(struct linux_binprm * bprm)
 		set_dumpable(current->mm, SUID_DUMP_USER);
 
 	perf_event_exec();
-	__set_task_comm(me, kbasename(bprm->filename), true);
+
+	/*
+	 * If fdpath was set, alloc_bprm() made up a path that will
+	 * probably not be useful to admins running ps or similar.
+	 * Let's fix it up to be something reasonable.
+	 */
+	if (bprm->fdpath) {
+		rcu_read_lock();
+		__set_task_comm(me, smp_load_acquire(&bprm->file->f_path.dentry->d_name.name),
+				true);
+		rcu_read_unlock();
+	}
+	else {
+		__set_task_comm(me, kbasename(bprm->filename), true);
+	}
 
 	/* An exec changes our domain. We are no longer part of the thread
 	   group */


I've minimally tested this.

-Kees

-- 
Kees Cook

Re: [GIT PULL] execve updates for v6.13-rc1

[Linus Torvalds]

On Thu, 21 Nov 2024 at 14:06, Kees Cook <kees@kernel.org> wrote:
>
> I think I finally figured out why you keep saying this. I think you mean
> to imply "ps -e" (or similar), not "ps". Asking for more process details
> ("ps a", "ps -f", "ps -e", etc) uses cmdline.

Ah. I never use plain 'ps'. The output is too useless.

> Since comm is mutable anyway, I feel like the "friendlier" default for
> userspace would be option 2.

The thing is, I still violently disagree.

I don't see what's "friendlier" in being (a) slower and (b) giving the
wrong output.

argv[0] isn't what we *normally* use.

And I've seen lots of cases where argv[0] is actually plain made-up garbage.

Christ, I went and looked at OUR OWN TEST-CASES, and they just happily
lie about "argv[0]".

Just go check tools/testing/selftests/exec/execveat.c, and see.

So  no. THERE IS NO WAY I WILL ACCEPT THE GARBAGE THAT IS ARGV[0].

What is so hard to understand about the fact that argv[0] has never
*EVER* been meaningful? We're not making it so now.

            Linus

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Thu, Nov 21, 2024 at 10:59:59PM -0600, Eric W. Biederman wrote:
> Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> writes:
> 
> > On Wed, Nov 20, 2024 at 06:23:55PM -0800, Linus Torvalds wrote:
> >> On Wed, 20 Nov 2024 at 16:55, Eric W. Biederman <ebiederm@xmission.com> wrote:
> >> >
> >> > __set_task_comm cannot be called with bprm->file->f_dentry
> >> > unconditionally.
> >> 
> >> No, no. Only for the "no path" case.
> >> 
> >> > The reason bprm->file->f_dentry.dentry was abandoned were concerns
> >> > about breaking userspace.
> >> 
> >> There's no way it can break user space considering that right now
> >> comm[] ends up being just garbage.
> >
> > It'll "break userspace" in the sense the the resulting program name
> > visible in /proc/self/{comm,stat,status} would be different than the
> > expected value.
> 
> This actually has not been shown.
> 
> In the general case of programs on linux it is indeed the case
> we have multi-call binaries and symlinks to binaries.
> 
> Limiting things to just fexecve reduces the scope,
> I didn't think about the scope reduction when you made this argument the
> first time.
> 
> I do not see any evidence that there are daemons started by systemd
> where systemd follows the name in /etc/alternatives on debian, or winds
> up following a symlink on a multicall binary.  The way I understand
> /etc/alternatives I don't think you would ever want to use it for the
> name of a daemon that is put in a unit file.

systemd-udevd is one example that ~everybody has installed.
(Though it doesn't use comm, it uses argv[0] to decide behaviour.)

We can certainly find more, on my Fedora system, 337/2252 files
in /usr/sbin/ are symlinks, quite a few candidates.
But even if we went through every one of those, it's not enough,
because people have custom unit files and there's also systemd-run
and run0.

> All of these cases where you get a task->comm that would be different
> with fexecve are rare oddball cases today.  I think it is quite likely
> nothing systemd wants to start with fexecve will have this problem.
> 
> Further you can detect this problem before you get as far as starting
> the application.  Just pass O_NOFOLLOW to open and you can refuse to
> follow the potentially confusing symlinks.

We'd have two choices: refuse the command, which is not really a great
option, or fall back to execve(), an approach which I find really
unappealing.

> So short of finding real programs in the real world that actually care
> it seems perfectly reasonable to do the cheap and easy thing.
>
> Right now it feels like you are adopting a very cautious approach and
> arguing for an expensive implementation simply because it is a lot of
> work to figure out what programs you actually care about and see what
> those programs are actually doing.

So it's not really "programs" per se. For example, systemd itself
doesn't care at all, because it uses cgroups to manage processes. I've
been using systemd-with-fexecve for two years and apart from strange 'ps'
listings, it's all fine. But we have administrators and scripts.
For example, 'pgrep systemd-udevd' and 'pkill -HUP httpd' and a
bazillion other calls that we cannot ever find or change.

> On the system I am writing this on right now there are about 300
> processes running and only about 17 whose parent is pid 1.  Most of
> those process whose parent is pid 1 run as root.  Which means extra care
> needs to be taken with them anyway.

This would also affect user process started by 'systemd --user',
i.e. most of the graphical user session nowadays (under GNOME, KDE,
etc.)

> > Even if we end up copying a string from userspace unnecessarilly,
> > does this matter? execve is a heavyweight operation and copying a
> > a dozen bytes extra hardly matters.
> 
> Generally it is on the person arguing for making the kernel more
> expensive to find a compelling case.  In this instance my imagination
> fails me in finding a binary that systemd will start that will be
> affected.  So I simply don't see the point in making the code
> more expensive.
> 
> I was really hoping we could use the cheap bprm->file->f_dentry
> to set task->comm in all cases as that would make the kernel
> simpler and faster.
> 
> To me using bprm->file->f_dentry still seems to make more sense than a
> field whose value (in the case of login shells) is documented as being
> different from what you are looking for.
> 
> There is no solution that doesn't have down sides.  As such the kernel
> might as well use a solution that is cheap and as close to how
> task->comm has been calculated historically as possible.

Right. It comes down to judgement / guesswork about the downsides of
each approach.

As I wrote before, the approach with bprm->file->f_dentry is _much_
better than what we have now. So I'll take that over status quo.
But from the POV of a distro maintainer, it's not ideal. Basically,
with the patch that uses argv[0], I expect almost no user-visible
change, so I can rebuild systemd with -Dfexecve=true in Fedora and
maybe send a mail to fedora-devel with "hey, watch out, the way
systemd starts processes has been changed, selinux could be affected,
please report any errors".

With the f_dentry version, we'll still do the transition, but we'll
need to make an effort to warn users and do it much more visibly, and
based on past experience with such things, there'll still inevitably
be users who report that their favourite backup script that they call
from the git checkout now doesn't terminate properly and stuff like
that. Every kernel maintainer knows that any aspect of behaviour that
is visibile to users starts being relied on…

I think fexecve is a nice feature (as all the fd-based syscalls
in general), but it need to be drop-in replacement if it's supposed
to be adopted quickly.

Zbyszek

Re: [GIT PULL] execve updates for v6.13-rc1

[Harald Arnesen]

Zbigniew Jędrzejewski-Szmek [2024-11-22 08:47:30]:

>> I do not see any evidence that there are daemons started by systemd
>> where systemd follows the name in /etc/alternatives on debian, or winds
>> up following a symlink on a multicall binary.  The way I understand
>> /etc/alternatives I don't think you would ever want to use it for the
>> name of a daemon that is put in a unit file.

> systemd-udevd is one example that ~everybody has installed.
> (Though it doesn't use comm, it uses argv[0] to decide behaviour.)

There are quite a few of us who do NOT have systemd-udevd (or anything 
systemd) installed.
-- 
Hilsen Harald

Re: [GIT PULL] execve updates for v6.13-rc1

[Zbigniew Jędrzejewski-Szmek]

On Fri, Nov 22, 2024 at 08:43:01AM -0600, Eric W. Biederman wrote:
> Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> writes:
> 
> > On Thu, Nov 21, 2024 at 10:59:59PM -0600, Eric W. Biederman wrote:
> >> Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> writes:
> >> 
> >> > On Wed, Nov 20, 2024 at 06:23:55PM -0800, Linus Torvalds wrote:
> >> >> On Wed, 20 Nov 2024 at 16:55, Eric W. Biederman <ebiederm@xmission.com> wrote:
> >> >> >
> >> >> > __set_task_comm cannot be called with bprm->file->f_dentry
> >> >> > unconditionally.
> >> >> 
> >> >> No, no. Only for the "no path" case.
> >> >> 
> >> >> > The reason bprm->file->f_dentry.dentry was abandoned were concerns
> >> >> > about breaking userspace.
> >> >> 
> >> >> There's no way it can break user space considering that right now
> >> >> comm[] ends up being just garbage.
> >> >
> >> > It'll "break userspace" in the sense the the resulting program name
> >> > visible in /proc/self/{comm,stat,status} would be different than the
> >> > expected value.
> >> 
> >> This actually has not been shown.
> >> 
> >> In the general case of programs on linux it is indeed the case
> >> we have multi-call binaries and symlinks to binaries.
> >> 
> >> Limiting things to just fexecve reduces the scope,
> >> I didn't think about the scope reduction when you made this argument the
> >> first time.
> >> 
> >> I do not see any evidence that there are daemons started by systemd
> >> where systemd follows the name in /etc/alternatives on debian, or winds
> >> up following a symlink on a multicall binary.  The way I understand
> >> /etc/alternatives I don't think you would ever want to use it for the
> >> name of a daemon that is put in a unit file.
> >
> > systemd-udevd is one example that ~everybody has installed.
> > (Though it doesn't use comm, it uses argv[0] to decide behaviour.)
> 
> As far as I can tell using task->comm is a bug.
> argv[0] is cheaper and the unix convention for deciding program
> behavior.

I don't understand what you mean by "a bug". As I wrote, systemd-udevd
does NOT use comm. And in other places, no matter what you or I might
wish, comm definitely IS used. Any place with pgrep or pkill is doing
exactly that. So an assertion that it's against a "unix convention" 
is unjustified.

[snip part about udev]

> It is worth remembering that hardlinks are going to be cheaper all
> around for this sort of thing.  The only reason I see for using
> symlinks is that you can see that something is an alias.

Yes. Symlinks are used so that that aliasing is visible.
The second reason is that symlinks can be used across devices,
so for example a symlink can be made from /etc/aliases to /usr,
which are sometimes on different devices.
The third reason is that when things are split between different
packages (in the sense of distro packages), a package can contain
multiple files that are hardlinked, and a package can contain a
symlink to a file in a different package (or even something that
is not packaged), but a package cannot natively provide a hardlink
to a file in a different package. (A post-install scriptlet could
make a hardlink, but that's much more work and preferrably avoided,
even if technically possible.)

> > We can certainly find more, on my Fedora system, 337/2252 files
> > in /usr/sbin/ are symlinks, quite a few candidates.
> > But even if we went through every one of those, it's not enough,
> > because people have custom unit files and there's also systemd-run
> > and run0.
> 
> >> All of these cases where you get a task->comm that would be different
> >> with fexecve are rare oddball cases today.  I think it is quite likely
> >> nothing systemd wants to start with fexecve will have this problem.
> >> 
> >> Further you can detect this problem before you get as far as starting
> >> the application.  Just pass O_NOFOLLOW to open and you can refuse to
> >> follow the potentially confusing symlinks.
> >
> > We'd have two choices: refuse the command, which is not really a great
> > option, or fall back to execve(), an approach which I find really
> > unappealing.
> 
> It also allows you to report the program and quickly find which cases
> might be a problem.



> >> So short of finding real programs in the real world that actually care
> >> it seems perfectly reasonable to do the cheap and easy thing.
> >>
> >> Right now it feels like you are adopting a very cautious approach and
> >> arguing for an expensive implementation simply because it is a lot of
> >> work to figure out what programs you actually care about and see what
> >> those programs are actually doing.
> >
> > So it's not really "programs" per se. For example, systemd itself
> > doesn't care at all, because it uses cgroups to manage processes. I've
> > been using systemd-with-fexecve for two years and apart from strange 'ps'
> > listings, it's all fine. But we have administrators and scripts.
> > For example, 'pgrep systemd-udevd' and 'pkill -HUP httpd' and a
> > bazillion other calls that we cannot ever find or change.
> 
> Hmm.
> 
> Not even my system where I run apache has a file named httpd
> so I don't have enough context to understand that reference.

It's a long-standing Red Hat vs. Debian naming war. Some want
"apache httpd" to be called "apache" and others "httpd". In the
Fedora package, we have '/usr/sbin/httpd'.

Apache/httpd was (is?) deployed in a few major versions, so it was
common to have httpd-2.2 and httpd-2.4 installed, and in those
situations one would create a symlink to "decide" which version
is the main one. This is the situation I was alluding to.

Another common example that comes to mind is python: distros provide
/usr/bin/python, /usr/bin/python2, and /usr/bin/python3 as symlinks
to specific versions. On my Fedora system, 
python-unversioned-command.rpm provides the /usr/bin/python → python3
symlink, and python3.13.rpm provides /usr/bin/python3.13 binary
and a /usr/bin/python3 → python3.13 symlink. So this is
an example of a cross-package "alias" that pretty much must
be a symlink.

> >> To me using bprm->file->f_dentry still seems to make more sense than a
> >> field whose value (in the case of login shells) is documented as being
> >> different from what you are looking for.
> >> 
> >> There is no solution that doesn't have down sides.  As such the kernel
> >> might as well use a solution that is cheap and as close to how
> >> task->comm has been calculated historically as possible.
> >
> > Right. It comes down to judgement / guesswork about the downsides of
> > each approach.
> >
> > As I wrote before, the approach with bprm->file->f_dentry is _much_
> > better than what we have now. So I'll take that over status quo.
> 
> I really think that is where the kernel should start.
> It is cheap, and it solves the issue that task->comm was set to
> something meaningless from a process standpoint.

Ack.

> > But from the POV of a distro maintainer, it's not ideal. Basically,
> > with the patch that uses argv[0], I expect almost no user-visible
> > change, so I can rebuild systemd with -Dfexecve=true in Fedora and
> > maybe send a mail to fedora-devel with "hey, watch out, the way
> > systemd starts processes has been changed, selinux could be affected,
> > please report any errors".
> 
> Huh?  If selinux depending upon filenames?  I think if any security
> module is somehow depending upon task->comm it should be fixed.
> Just because task->comm very much is not built so that it can
> be depended on that way.

I never said anything about selinux depending on comm.
Selinux is primarily based on syscalls, so changing execve() to
execveat() _could_ require a change to some policies. (I'm not saying
that it must. I didn't explore the topic. Just that it's something
that'd need to be checked.)

> > I think fexecve is a nice feature (as all the fd-based syscalls
> > in general), but it need to be drop-in replacement if it's supposed
> > to be adopted quickly.
> 
> I don't know why you want to use fexecve.  Using an fd based system
> call will typically be slower.  Using fd-based system calls is generally
> what you want if there are extra checks you want to add before
> performing an action.  Opening and file and then calling exec is going
> to be necessarily slower than letting execve handle it all.  Not to
> mention the cost of the added checks.

That's exactly the reason. We do various checks on the binary before
calling it, for example SMACK context setup. Another example is that
we've been working on adding a .note.package section [1] that uniquely
identifies the provenience of ELF files. I plan to add a feature where
we read the ELF note and log the specific version of the program being
launched. It's more elegant to open a file and use the same fd to do
all the preparatory steps without a change of TOCTOU race, for example
if packages are being upgraded and the file might be replaced in the
background.

[1] https://systemd.io/ELF_PACKAGE_METADATA/

> Depending on the checks you want to perform in userspace to the
> executing of a binary it is possible to use a dirfd with execveat
> instead of the fd of the actual binary.  That allows you to have enough
> of the full pathname that task->comm will be set the same.

Yeah, that's not what we want. A package upgrade will use an
atomic rename to replace files within the directory.

Zbyszek