Re: [PATCH 4/7] fs/binfmt_elf: use get_task_comm instead of open-coded string copy

From: Petr Mladek
Date: Thu Nov 11 2021 - 06:35:07 EST


On Thu 2021-11-11 11:06:04, David Hildenbrand wrote:
> On 11.11.21 11:03, David Hildenbrand wrote:
> > On 08.11.21 09:38, Yafang Shao wrote:
> >> It is better to use get_task_comm() instead of the open coded string
> >> copy as we do in other places.
> >>
> >> struct elf_prpsinfo is used to dump the task information in userspace
> >> coredump or kernel vmcore. Below is the verfication of vmcore,
> >>
> >> crash> ps
> >> PID PPID CPU TASK ST %MEM VSZ RSS COMM
> >> 0 0 0 ffffffff9d21a940 RU 0.0 0 0 [swapper/0]
> >>> 0 0 1 ffffa09e40f85e80 RU 0.0 0 0 [swapper/1]
> >>> 0 0 2 ffffa09e40f81f80 RU 0.0 0 0 [swapper/2]
> >>> 0 0 3 ffffa09e40f83f00 RU 0.0 0 0 [swapper/3]
> >>> 0 0 4 ffffa09e40f80000 RU 0.0 0 0 [swapper/4]
> >>> 0 0 5 ffffa09e40f89f80 RU 0.0 0 0 [swapper/5]
> >> 0 0 6 ffffa09e40f8bf00 RU 0.0 0 0 [swapper/6]
> >>> 0 0 7 ffffa09e40f88000 RU 0.0 0 0 [swapper/7]
> >>> 0 0 8 ffffa09e40f8de80 RU 0.0 0 0 [swapper/8]
> >>> 0 0 9 ffffa09e40f95e80 RU 0.0 0 0 [swapper/9]
> >>> 0 0 10 ffffa09e40f91f80 RU 0.0 0 0 [swapper/10]
> >>> 0 0 11 ffffa09e40f93f00 RU 0.0 0 0 [swapper/11]
> >>> 0 0 12 ffffa09e40f90000 RU 0.0 0 0 [swapper/12]
> >>> 0 0 13 ffffa09e40f9bf00 RU 0.0 0 0 [swapper/13]
> >>> 0 0 14 ffffa09e40f98000 RU 0.0 0 0 [swapper/14]
> >>> 0 0 15 ffffa09e40f9de80 RU 0.0 0 0 [swapper/15]
> >>
> >> It works well as expected.
> >>
> >> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> >> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> >> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> >> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> >> Cc: Michal Miroslaw <mirq-linux@xxxxxxxxxxxx>
> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> >> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> >> Cc: Petr Mladek <pmladek@xxxxxxxx>
> >> ---
> >> fs/binfmt_elf.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> >> index a813b70f594e..138956fd4a88 100644
> >> --- a/fs/binfmt_elf.c
> >> +++ b/fs/binfmt_elf.c
> >> @@ -1572,7 +1572,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
> >> SET_UID(psinfo->pr_uid, from_kuid_munged(cred->user_ns, cred->uid));
> >> SET_GID(psinfo->pr_gid, from_kgid_munged(cred->user_ns, cred->gid));
> >> rcu_read_unlock();
> >> - strncpy(psinfo->pr_fname, p->comm, sizeof(psinfo->pr_fname));
> >> + get_task_comm(psinfo->pr_fname, p);
> >>
> >> return 0;
> >> }
> >>
> >
> > We have a hard-coded "pr_fname[16]" as well, not sure if we want to
> > adjust that to use TASK_COMM_LEN?
>
> But if the intention is to chance TASK_COMM_LEN later, we might want to
> keep that unchanged.

It seems that len will not change in the end. Another solution is
going to be used for the long names, see
https://lore.kernel.org/r/20211108084142.4692-1-laoar.shao@xxxxxxxxx.

> (replacing the 16 by a define might still be a good idea, similar to how
> it's done for ELF_PRARGSZ, but just a thought)

If the code would need some tweaking when the size changes, you could
still use TASK_COMM_LEN and trigger a compilation error when the size
gets modified. For example, static_assert(TASK_COMM_LEN == 16);

It will make it clear that it needs attention if the size is ever modified.

Best Regards,
Petr