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SubjectRe: Documenting execve() and EAGAIN
Hello Vasiliy,

On 05/26/2014 08:11 PM, Vasiliy Kulikov wrote:
> Hi Michael,
>
> On Wed, May 21, 2014 at 20:12 +0200, Michael Kerrisk (man-pages) wrote:
>> Vasily (and Motohiro),
>>
>> Sometime ago, Motohiro raised a documentation bug
>> ( https://bugzilla.kernel.org/show_bug.cgi?id=42704 ) which
>> relates to your commit 72fa59970f8698023045ab0713d66f3f4f96945c
>> ("move RLIMIT_NPROC check from set_user() to do_execve_common()")
>>
>> I have attempted to document this, and I would like to ask you
>> (and Motohiro) if you would review the text proposed below for
>> the exceve(2) man page.
>>
>> Thank you,
>>
>> Michael
>>
>>
>> ERRORS
>> EAGAIN (since Linux 3.1)
>> Having changed its real UID using one of the set*uid()
>> calls, the caller was—and is now still—above its
>> RLIMIT_NPROC resource limit (see setrlimit(2)). For a
>> more detailed explanation of this error, see NOTES.
>>
>> NOTES
>> execve() and EAGAIN
>> A more detailed explanation of the EAGAIN error that can occur
>> (since Linux 3.1) when calling execve() is as follows.
>>
>> The EAGAIN error can occur when a preceding call to setuid(2),
>> setreuid(2), or setresuid(2) caused the real user ID of the
>> process to change, and that change caused the process to
>> exceed its RLIMIT_NPROC resource limit (i.e., the number of
>> processes belonging to the new real UID exceeds the resource
>> limit). In Linux 3.0 and earlier, this caused the set*uid()
>> call to fail.
>>
>> Since Linux 3.1, the scenario just described no longer causes
>> the set*uid() call to fail, because it too often led to secu‐
>> rity holes because buggy applications didn't check the return
>> status and assumed that—if the caller had root privileges—the
>> call would always succeed. Instead, the set*uid() calls now
>> successfully change real UID, but the kernel sets an internal
>> flag, named PF_NPROC_EXCEEDED, to note that the RLIMIT_NPROC
>> resource limit has been exceeded. If the resource limit is
>> still exceeded at the time of a subsequent execve() call, that
>> call fails with the error EAGAIN. This kernel logic ensures
>> that the RLIMIT_NPROC resource limit is still enforced for the
>> common privileged daemon workflow—namely, fork(2)+ set*uid()+
>> execve(2).
>>
>> If the resource limit was not still exceeded at the time of
>> the execve() call (because other processes belonging to this
>> real UID terminated between the set*uid() call and the
>> execve() call), then the execve() call succeeds and the kernel
>> clears the PF_NPROC_EXCEEDED process flag. The flag is also
>> cleared if a subsequent call to fork(2) by this process suc‐
>> ceeds.
>
> Probably explicitly state that NPROC check on execve() is processed only
> in case of a previous set*uid() call? If there was no previous
> set*uid() call the semantics of execve() checks are the same as before
> (IOW, RLIMIT_NPROC is ignored).

Yes, good idea. I'll add some words to make that clearer.

> The rest is fine.

Thanks for checking it!

Cheers,

Michael


--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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