[PATCH 0/2] proc.5: note broken v4.18 userspace promise

[PATCH 0/2] proc.5: note broken v4.18 userspace promise

[Ævar Arnfjörð Bjarmason]

Along with a trivial change in 1/2 (which would otherwise textually
conflict) this 2/2 notes that since Linux 4.18 the promise that the
"comm" field in /proc/PID/stat will be no longer than 15 characters
hasn't been true for the "kworker" processes.

This caveat was noted in a discussion on HN
https://news.ycombinator.com/item?id=34093845;

I myself have code in git/git@2d3491b117c (tr2: log N parent process
names on Linux, 2021-08-27) which won't do anything bad if this were
to be combined with PID recycling (with a kernel thread usurping the
PID of a process that used to belong to our parent), but it will
behave unexpectedly. I wrote that code against the promises made in
proc(5) at the time.

As I'm about to send this I notice that [1] was sent yesterday, which
textually conflicts with this submission[1]. I've added its authors to
the CC (I'm not on the linux-man list).

Personally I never read the note that the "comm" string would be
contained in parentheses as a promise that the kernel was going to
strip ")" from userspace names, or only allow balanced parentheses or
whatever.

I'd think most programmers would see a mention of a \0-delimited
string and assume that it would contain anything but "\0" (as is the
case here). Perhaps it's useful to some to include such a clarifying
blurb, but I personally find it superfluous.

Whereas the fix here is a fix for a promise we're currently making
which hasn't been true since v4.18.

1. https://lore.kernel.org/linux-man/Y6SJDbKBk471KE4k@p183

Ævar Arnfjörð Bjarmason (2):
  proc.5: note that "cmdline" might be favored over "stat.comm" by ps(1)
  proc.5: the "comm" field can be longer than 16 bytes

 man5/proc.5 | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

-- 
2.39.0.1106.gf45ba805d1a

[PATCH 1/2] proc.5: note that "cmdline" might be favored over "stat.comm" by ps(1)

[Ævar Arnfjörð Bjarmason]

With "ps -A" the output uses the "stat.comm" field, but "ps a" will
display the value in "cmdline".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 man5/proc.5 | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/man5/proc.5 b/man5/proc.5
index 65a4c38e3..115c8592e 100644
--- a/man5/proc.5
+++ b/man5/proc.5
@@ -2087,7 +2087,11 @@ The affected fields are indicated with the marking [PT].
 The process ID.
 .TP
 (2) \fIcomm\fP \ %s
-The filename of the executable, in parentheses.
+The filename of the executable, in parentheses. Tools such as
+.BR ps (1)
+may alternatively (or additionally) use
+.IR /proc/ pid /cmdline.
+.IP
 Strings longer than
 .B TASK_COMM_LEN
 (16) characters (including the terminating null byte) are silently truncated.
-- 
2.39.0.1106.gf45ba805d1a

Re: [PATCH 1/2] proc.5: note that "cmdline" might be favored over "stat.comm" by ps(1)

[Alejandro Colomar]

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Hi Ævar,

On 12/23/22 18:59, Ævar Arnfjörð Bjarmason wrote:
> With "ps -A" the output uses the "stat.comm" field, but "ps a" will
> display the value in "cmdline".
> 
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>   man5/proc.5 | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/man5/proc.5 b/man5/proc.5
> index 65a4c38e3..115c8592e 100644
> --- a/man5/proc.5
> +++ b/man5/proc.5
> @@ -2087,7 +2087,11 @@ The affected fields are indicated with the marking [PT].
>   The process ID.
>   .TP
>   (2) \fIcomm\fP \ %s
> -The filename of the executable, in parentheses.
> +The filename of the executable, in parentheses. Tools such as

Please use semantic newlines.  See man-pages(7):

    Use semantic newlines
        In  the source of a manual page, new sentences should be started on new
        lines, long sentences should be split into lines at clause breaks (com‐
        mas, semicolons, colons, and so on), and long clauses should  be  split
        at  phrase  boundaries.   This convention, sometimes known as "semantic
        newlines", makes it easier to see the effect of  patches,  which  often
        operate at the level of individual sentences, clauses, or phrases.


Cheers,

Alex

> +.BR ps (1)
> +may alternatively (or additionally) use
> +.IR /proc/ pid /cmdline.
> +.IP
>   Strings longer than
>   .B TASK_COMM_LEN
>   (16) characters (including the terminating null byte) are silently truncated.

-- 
<http://www.alejandro-colomar.es/>

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[PATCH 2/2] proc.5: the "comm" field can be longer than 16 bytes

[Ævar Arnfjörð Bjarmason]

Since torvalds/linux@6b59808bfe48 (workqueue: Show the latest
workqueue name in /proc/PID/{comm,stat,status}, 2018-05-18) the limit
of 15 character comm names hasn't applied to "kworker" processes. This
can be seen e.g. on my Linux v5.10 box:

	$ awk '{print $2}' /proc/*/stat 2>/dev/null | grep kworker  | sort -R | head -n 5
	(kworker/3:1-mm_percpu_wq)
	(kworker/0:0H-events_highpri)
	(kworker/1:1H-kblockd)
	(kworker/u16:1-events_unbound)
	(kworker/u17:0)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 man5/proc.5 | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/man5/proc.5 b/man5/proc.5
index 115c8592e..b23dd1479 100644
--- a/man5/proc.5
+++ b/man5/proc.5
@@ -2092,9 +2092,13 @@ The filename of the executable, in parentheses. Tools such as
 may alternatively (or additionally) use
 .IR /proc/ pid /cmdline.
 .IP
-Strings longer than
+For userspace, strings longer than
 .B TASK_COMM_LEN
 (16) characters (including the terminating null byte) are silently truncated.
+Since Linux version 4.18.0 a longer limit of 64 (including the
+terminating null byte) has applied to the kernel's own workqueue
+workers (whose names start with "kworker/").
+.IP
 This is visible whether or not the executable is swapped out.
 .TP
 (3) \fIstate\fP \ %c
-- 
2.39.0.1106.gf45ba805d1a

Re: [PATCH 0/2] proc.5: note broken v4.18 userspace promise

[Linus Torvalds]

On Fri, Dec 23, 2022 at 10:00 AM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
>
> Whereas the fix here is a fix for a promise we're currently making
> which hasn't been true since v4.18.

Hah. Ack. Did anybody ever actually notice?

I wonder if the newer limit of 64 characters for kworkers shouldn't
even be mentioned at all, and if the 16-byte truncation for user space
should also be just removed.

Those limits should never have been some documented API, they were
always just implementation details, after all.

             Linus

Re: [PATCH 0/2] proc.5: note broken v4.18 userspace promise

[Alejandro Colomar]

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Hi,

On 12/23/22 19:12, Linus Torvalds wrote:
> On Fri, Dec 23, 2022 at 10:00 AM Ævar Arnfjörð Bjarmason
> <avarab@gmail.com> wrote:
>>
>> Whereas the fix here is a fix for a promise we're currently making
>> which hasn't been true since v4.18.
> 
> Hah. Ack. Did anybody ever actually notice?
> 
> I wonder if the newer limit of 64 characters for kworkers shouldn't
> even be mentioned at all, and if the 16-byte truncation for user space
> should also be just removed.
> 
> Those limits should never have been some documented API, they were
> always just implementation details, after all.
> 
>               Linus


I agree.  A variable implementation detail like this doesn't provide anything 
valuable to users; especially since there's no statbility promise at all.  I'd 
rewrite to just remove the (16) implementation detail.

Ævar, would you send an v2 that removes implementation details, rather than 
fixing the details?

Thanks for the patch set!

Cheers,

Alex

On 12/23/22 18:59, Ævar Arnfjörð Bjarmason wrote:
 > diff --git a/man5/proc.5 b/man5/proc.5
 > index 115c8592e..b23dd1479 100644
 > --- a/man5/proc.5
 > +++ b/man5/proc.5
 > @@ -2092,9 +2092,13 @@ The filename of the executable, in parentheses. Tools 
such as
 >   may alternatively (or additionally) use
 >   .IR/proc/  pid /cmdline.
 >   .IP
 > -Strings longer than
 > +For userspace, strings longer than
 >   .B TASK_COMM_LEN
 >   (16) characters (including the terminating null byte) are silently truncated.
 > +Since Linux version 4.18.0 a longer limit of 64 (including the
 > +terminating null byte) has applied to the kernel's own workqueue
 > +workers (whose names start with "kworker/").
 > +.IP
 >   This is visible whether or not the executable is swapped out.
 >   .TP
 >   (3) \fIstate\fP \ %c

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 0/2] proc.5: note broken v4.18 userspace promise

[Ævar Arnfjörð Bjarmason]

On Wed, Dec 28 2022, Alejandro Colomar wrote:

> [[PGP Signed Part:Undecided]]
> Hi,
>
> On 12/23/22 19:12, Linus Torvalds wrote:
>> On Fri, Dec 23, 2022 at 10:00 AM Ævar Arnfjörð Bjarmason
>> <avarab@gmail.com> wrote:
>>>
>>> Whereas the fix here is a fix for a promise we're currently making
>>> which hasn't been true since v4.18.
>> Hah. Ack. Did anybody ever actually notice?
>> I wonder if the newer limit of 64 characters for kworkers shouldn't
>> even be mentioned at all, and if the 16-byte truncation for user space
>> should also be just removed.
>> Those limits should never have been some documented API, they were
>> always just implementation details, after all.
>>               Linus
>

Sorry about the late reply, holidays.

> I agree.  A variable implementation detail like this doesn't provide
> anything valuable to users; especially since there's no statbility
> promise at all.  I'd rewrite to just remove the (16) implementation
> detail.
>
> Ævar, would you send an v2 that removes implementation details, rather
> than fixing the details?

Maybe, because I'm not sure I'm qualified to document this anymore. My
current patch just extends the description to cover the 4.18 divergence.

Let's separate a few things here:

 A. The long-standing docs promise that it's limited to 16 bytes
 B. Since 4.18 that hasn't been true for (some of) the kernel's own
    processes, where the limit's been 64.
 C. Was the part of "A" where a limit was documented at all a good idea
    in retrospect?
 D. If "C"'s a "no" (which seems to be the consensus) what should the
    docs say?
 E. I hadn't mentioned this before, but the docs for prctl()'s
    PR_SET_NAME document the same 16 byte limit.

I think the current behavior since 4.18 is a broken userspace promise,
although admittedly a minor/obscure one.

I think even if going forward the documentation is deliberately
ambiguous about it, it would make sense to briefly document the 16 and
64 byte limits as past limits, to at least help to explain why current
code parsing "/proc/*/stat" seems to be confident in those (or more
commonly, the 16 bytes).

The code I wrote was rather anal about that promise, but e.g. looking at
htop(1)'s source code they've got a total limit of 2048 for this sort of
line (MAX_READ). I'm sure if I went fishing I could find other similar
cases (and probably some lower ones).

I don't think it would be good to just leave it ambiguous for those
trying to use this interface. They might assume any of 16 bytes (from
finding the prctl() PR_SET_NAME docs), 64 bytes (from reading kernel
sources), 255 (maximum filename length) etc.

Wouldn't the least bad thing be to:

 * Cover "A" and "B" in passing, i.e. explain past promised /
   implemented limits.
 * Note that this is no guarantee, but that...
 * ...we might use up to N, where N is some sane limit (1024? 2048?
   4096?).

   So programs that parse this now could just increase their fixed
   buffers, rather than having to use some getc()/realloc() loop, as
   they might if the interface makes no promises about an upper bound,
   and if they're being paranoid about future-proofing the parser.

   If so I have no opinion on what value of "N" would be sane, other
   than it seems best to pick something.

?

> On 12/23/22 18:59, Ævar Arnfjörð Bjarmason wrote:
>> diff --git a/man5/proc.5 b/man5/proc.5
>> index 115c8592e..b23dd1479 100644
>> --- a/man5/proc.5
>> +++ b/man5/proc.5
>> @@ -2092,9 +2092,13 @@ The filename of the executable, in
>   parentheses. Tools such as
>>   may alternatively (or additionally) use
>>   .IR/proc/  pid /cmdline.
>>   .IP
>> -Strings longer than
>> +For userspace, strings longer than
>>   .B TASK_COMM_LEN
>>   (16) characters (including the terminating null byte) are silently truncated.
>> +Since Linux version 4.18.0 a longer limit of 64 (including the
>> +terminating null byte) has applied to the kernel's own workqueue
>> +workers (whose names start with "kworker/").
>> +.IP
>>   This is visible whether or not the executable is swapped out.
>>   .TP
>>   (3) \fIstate\fP \ %c

Re: [PATCH 0/2] proc.5: note broken v4.18 userspace promise

[Tejun Heo]

On Fri, Dec 23, 2022 at 10:12:22AM -0800, Linus Torvalds wrote:
> On Fri, Dec 23, 2022 at 10:00 AM Ævar Arnfjörð Bjarmason
> <avarab@gmail.com> wrote:
> >
> > Whereas the fix here is a fix for a promise we're currently making
> > which hasn't been true since v4.18.
> 
> Hah. Ack. Did anybody ever actually notice?
> 
> I wonder if the newer limit of 64 characters for kworkers shouldn't
> even be mentioned at all, and if the 16-byte truncation for user space
> should also be just removed.
> 
> Those limits should never have been some documented API, they were
> always just implementation details, after all.

In 2018, 6b59808bfe48 ("workqueue: Show the latest workqueue name in
/proc/PID/{comm,stat,status}") extended the length of the wq workers. 64
used for the formatting buffer size is definitely an implementation detail.

Last year, Yafang (cc'd) added d6986ce24fc0 ("kthread: dynamically allocate
memory to store kthread's full name") removed restrictions on kthread names
completely. It now gets dynamically allocated if long than TASK_COMM_LEN.
BTW, on allocation failure, the name gets silently truncated. I think it'd
be better to fail kthread creation instead (not that we're likely to fail
these allocations but still).

I think the right thing to do here is saying that all kernel thread names
can be longer than 16.

Thanks.

-- 
tejun