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SubjectRe: [PATCH] pidfd: fix test failure due to stack overflow on some arches
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On 1/28/22 1:56 AM, Christian Brauner wrote:
> On Thu, Jan 27, 2022 at 01:29:51PM -0800, Axel Rasmussen wrote:
>> When running the pidfd_fdinfo_test on arm64, it fails for me. After some
>> digging, the reason is that the child exits due to SIGBUS, because it
>> overflows the 1024 byte stack we've reserved for it.
>>
>> To fix the issue, increase the stack size to 8192 bytes (this number is
>> somewhat arbitrary, and was arrived at through experimentation -- I kept
>> doubling until the failure no longer occurred).
>>
>> Also, let's make the issue easier to debug. wait_for_pid() returns an
>> ambiguous value: it may return -1 in all of these cases:
>>
>> 1. waitpid() itself returned -1
>> 2. waitpid() returned success, but we found !WIFEXITED(status).
>> 3. The child process exited, but it did so with a -1 exit code.
>>
>> There's no way for the caller to tell the difference. So, at least log
>> which occurred, so the test runner can debug things.
>>
>> While debugging this, I found that we had !WIFEXITED(), because the
>> child exited due to a signal. This seems like a reasonably common case,
>> so also print out whether or not we have WIFSIGNALED(), and the
>> associated WTERMSIG() (if any). This lets us see the SIGBUS I'm fixing
>> clearly when it occurs.
>>
>> Finally, I'm suspicious of allocating the child's stack on our stack.
>> man clone(2) suggests that the correct way to do this is with mmap(),
>> and in particular by setting MAP_STACK. So, switch to doing it that way
>> instead.
>
> Heh, yes. :)
>
> commit 99c3a000279919cc4875c9dfa9c3ebb41ed8773e
> Author: Michael Kerrisk <mtk.manpages@gmail.com>
> Date: Thu Nov 14 12:19:21 2019 +0100
>
> clone.2: Allocate child's stack using mmap(2) rather than malloc(3)
>
> Christian Brauner suggested mmap(MAP_STACKED), rather than
> malloc(), as the canonical way of allocating a stack for the
> child of clone(), and Jann Horn noted some reasons why:
>
> Not on Linux, but on OpenBSD, they do use MAP_STACK now
> AFAIK; this was announced here:
> <http://openbsd-archive.7691.n7.nabble.com/stack-register-checking-td338238.html>.
> Basically they periodically check whether the userspace
> stack pointer points into a MAP_STACK region, and if not,
> they kill the process. So even if it's a no-op on Linux, it
> might make sense to advise people to use the flag to improve
> portability? I'm not sure if that's something that belongs
> in Linux manpages.
>
> Another reason against malloc() is that when setting up
> thread stacks in proper, reliable software, you'll probably
> want to place a guard page (in other words, a 4K PROT_NONE
> VMA) at the bottom of the stack to reliably catch stack
> overflows; and you probably don't want to do that with
> malloc, in particular with non-page-aligned allocations.
>
> And the OpenBSD 6.5 manual pages says:
>
> MAP_STACK
> Indicate that the mapping is used as a stack. This
> flag must be used in combination with MAP_ANON and
> MAP_PRIVATE.
>
> And I then noticed that MAP_STACK seems already to be on
> FreeBSD for a long time:
>
> MAP_STACK
> Map the area as a stack. MAP_ANON is implied.
> Offset should be 0, fd must be -1, and prot should
> include at least PROT_READ and PROT_WRITE. This
> option creates a memory region that grows to at
> most len bytes in size, starting from the stack
> top and growing down. The stack top is the start‐
> ing address returned by the call, plus len bytes.
> The bottom of the stack at maximum growth is the
> starting address returned by the call.
>
> The entire area is reserved from the point of view
> of other mmap() calls, even if not faulted in yet.
>
> Reported-by: Jann Horn <jannh@google.com>
> Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
>
>
>>
>> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
>> ---
>
> Yeah, stack handling - especially with legacy clone() - is yucky on the
> best of days. Thank you for the fix.
>
> Acked-by: Christian Brauner <brauner@kernel.org>
>

Thank you both. Will apply for 5.17-rc4 or so.

thanks,
-- Shuah

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