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Re: Dummy pthread functions in libc considered harmful


On 08/31/2015 10:59 AM, Rich Felker wrote:
> On Mon, Aug 31, 2015 at 05:54:46PM +0300, Alexander Monakov wrote:
>> On Tue, 25 Aug 2015, Rich Felker wrote:
>>>> (however when libpthread is not linked in, you know that pthread_mutex_lock is
>>>> not operating on shared memory: you'd need pthread_mutexattr_setpshared, which
>>>> is not provided in libc.so.6)
>>>
>>> This is false. The process that maps and locks the process-shared
>>> mutex need not be the one that called pthread_mutex_init. I ran into
>>> this issue when optimizing static linking; you can't omit
>>> process-shared code when the program doesn't reference
>>> pthread_mutexattr_setpshared, because it could get the mutex from
>>> another program -- that's the whole point of having it be
>>> process-shared.
>>
>> As a result, a libc "stub" mutex must:
>>
>>  - contain a full implementation for process-shared mutexes
>>  - for non-shared mutexes in single-threaded processes, modify them
>>    consistently with non-stub implementation (but modifications need not be
>>    atomic)
>>
>> Does Glibc do the former? (the bug report is about Glibc not doing the latter)
> 
> I may be wrong, but my impression is that glibc does not support this
> usage for process-shared mutexes (omitting -lpthread) and is relying
> on the POSIX rule that you may need -lpthread to get pthread_*
> functions, and interpreting this to mean that it's okay to have a
> broken/non-conforming version of pthread_mutex_* in libc.so. If so, I
> think this is technically correct, but risky since applications may be
> assuming that, if the symbol exists (e.g. via configure checks) then
> it works.

Correct, we consider the test case invalid because you failed to
link with -lpthread, and call it a QoI issue.

The problem is that it's a dangerous QoI issue that we should fix.

c.


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