Messages in this thread | | | Subject | Re: [RFC][PATCH] Replacing the /proc/<pid|self>/exe symlink code | From | Matt Helsley <> | Date | Thu, 31 May 2007 10:56:14 -0700 |
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On Wed, 2007-05-30 at 13:09 -0500, Serge E. Hallyn wrote: > Quoting Matt Helsley (matthltc@us.ibm.com): > > This patch avoids holding the mmap semaphore while walking VMAs in response to > > programs which read or follow the /proc/<pid|self>/exe symlink. This also allows us > > to merge mmu and nommu proc_exe_link() functions. The costs are holding a separate > > reference to the executable file stored in the task struct and increased code in > > fork, exec, and exit paths. > > > > Signed-off-by: Matt Helsley <matthltc@us.ibm.com> > > --- > > > > Compiled and passed simple tests for regressions when patched against a 2.6.20 > > and 2.6.22-rc2-mm1 kernel. > > > > fs/exec.c | 5 +++-- > > fs/proc/base.c | 20 ++++++++++++++++++++ > > fs/proc/internal.h | 1 - > > fs/proc/task_mmu.c | 34 ---------------------------------- > > fs/proc/task_nommu.c | 34 ---------------------------------- > > include/linux/sched.h | 1 + > > kernel/exit.c | 2 ++ > > kernel/fork.c | 10 +++++++++- > > 8 files changed, 35 insertions(+), 72 deletions(-)
<snip>
> > Index: linux-2.6.22-rc2-mm1/kernel/exit.c > > =================================================================== > > --- linux-2.6.22-rc2-mm1.orig/kernel/exit.c > > +++ linux-2.6.22-rc2-mm1/kernel/exit.c > > @@ -924,10 +924,12 @@ fastcall void do_exit(long code) > > if (unlikely(tsk->audit_context)) > > audit_free(tsk); > > > > taskstats_exit(tsk, group_dead); > > > > + if (tsk->exe_file) > > + fput(tsk->exe_file); > > Hi, > > just taking a cursory look so I may be missing something, but doesn't > this leave the possibility that right here, with tsk->exe_file being > put, another task would try to look at tsk's /proc/tsk->pid/exe? > > thanks, > -serge > > exit_mm(tsk); >
<snip>
Good question. To be precise, I think the problem doesn't exist here but after the exit_mm() because there's a VMA that holds a reference to the same file.
The existing code appears to solve the race between reading/following /proc/tsk->pid/exe and exit_mm() in the exit path by returning -ENOENT for the case where there is no executable VMA with a reference to the file backing it.
So I need to put NULL in the exe_file field and adjust the return value to be -ENOENT instead of -ENOSYS.
Thanks for the review!
Cheers, -Matt Helsley
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