RE: [PATCH 13/14] d_path: prepend_path() is unlikely to return non-zero

Justin He
2021-06-28T03:28:44Z

Hi Al

> -----Original Message-----
> From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: Saturday, June 26, 2021 1:59 AM
> To: Justin He <Justin.He@arm.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>; Petr Mladek
> <pmladek@suse.com>; Steven Rostedt <rostedt@goodmis.org>; Sergey
> Senozhatsky <senozhatsky@chromium.org>; Andy Shevchenko
> <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
> <linux@rasmusvillemoes.dk>; Jonathan Corbet <corbet@lwn.net>; Heiko
> Carstens <hca@linux.ibm.com>; Vasily Gorbik <gor@linux.ibm.com>; Christian
> Borntraeger <borntraeger@de.ibm.com>; Eric W . Biederman
> <ebiederm@xmission.com>; Darrick J. Wong <darrick.wong@oracle.com>; Peter
> Zijlstra (Intel) <peterz@infradead.org>; Ira Weiny <ira.weiny@intel.com>;
> Eric Biggers <ebiggers@google.com>; Ahmed S. Darwish
> <a.darwish@linutronix.de>; open list:DOCUMENTATION <linux-
> doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
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> fsdevel <linux-fsdevel@vger.kernel.org>
> Subject: Re: [PATCH 13/14] d_path: prepend_path() is unlikely to return
> non-zero
>
> On Fri, Jun 25, 2021 at 08:00:49AM +0000, Justin He wrote:
> > --- a/fs/d_path.c
> > +++ b/fs/d_path.c
> > @@ -210,6 +210,7 @@ static int prepend_path(const struct path *path,
> > b = *p;
> > read_seqbegin_or_lock(&rename_lock, &seq);
> > error = __prepend_path(path->dentry, real_mount(path->mnt), root,
> &b);
> > + printk("prepend=%d",error);
> > if (!(seq & 1))
> > rcu_read_unlock();
> > if (need_seqretry(&rename_lock, seq)) {
> >
> > Then the result seems a little different:
> > root@entos-ampere-02:~# dmesg |grep prepend=1 |wc -l
> > 7417
> > root@entos-ampere-02:~# dmesg |grep prepend=0 |wc -l
> > 772
> >
> > The kernel is 5.13.0-rc2+ + this series + my '%pD' series
> >
> > Any thoughts?
>
> On which loads? 1 here is "mount/dentry pair is in somebody
> else's namespace or outside of the subtree we are chrooted
> into". IOW, what's calling d_path() on your setup?

No special loads, merely collecting the results after kernel boots up.

To narrow down, I tested your branch [1] *without* my '%pD' series:
[1] https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/log/?h=work.d_path

The result is the same after kernel boots up.

The call trace are as follows:
The prepend_path returns 1:
Call trace:
prepend_path+0x144/0x340
d_absolute_path+0x6c/0xb8
aa_path_name+0x1c0/0x3d8
profile_transition+0x90/0x908
apparmor_bprm_creds_for_exec+0x914/0xaf0
security_bprm_creds_for_exec+0x34/0x50
bprm_execve+0x178/0x668
do_execveat_common.isra.47+0x1b4/0x1c8
__arm64_sys_execve+0x44/0x58
invoke_syscall+0x54/0x110
el0_svc_common.constprop.2+0x5c/0xe0
do_el0_svc+0x34/0xa0
el0_svc+0x20/0x30
el0_sync_handler+0x88/0xb0
el0_sync+0x148/0x180

The prepend_path returns 0:
Call trace:
prepend_path+0x144/0x340
d_path+0x110/0x158
proc_pid_readlink+0xbc/0x1b8
vfs_readlink+0x14c/0x170
do_readlinkat+0x134/0x168
__arm64_sys_readlinkat+0x28/0x38
invoke_syscall+0x54/0x110
el0_svc_common.constprop.2+0x5c/0xe0
do_el0_svc+0x34/0xa0
el0_svc+0x20/0x30
el0_sync_handler+0x88/0xb0
el0_sync+0x148/0x180

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