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Re: [PATCH v2 2/3] posix: Use posix_spawn on popen
On 24/10/2018 07:07, Florian Weimer wrote:
> * Adhemerval Zanella:
>
>> Changed from previous version:
>>
>> - Added note about BZ#17490 fix.
>> - Use proc_file_chain_lock to access proc_file_chain (BZ#22834).
>>
>> ---
>>
>> This patch uses posix_spawn on popen instead of fork and execl. On Linux
>> this has the advantage of much lower memory consumption (usually 32 Kb
>> minimum for the mmap stack area).
>
> Does this need a NEWS entry, noting the fork handler aspect?
Although BZ#17490 already tracks it, including a discussion about possible
POSIX deviation, it won't hurt. What about:
* The popen and system do not run atfork handlers anymore (BZ#17490).
Although it is a possible POSIX violation, the POSIX rationale in
pthread_atfork documentation regarding atfork handlers is to handle
incosistent mutex state after fork call in multithread environment. In
both popen and system there is no direct access to user defined mutexes.
As a side-note, we also changed posix_spawn behaviour regarding atfork
handler (at least when it used fork instead of vfork) with its new
implementation.
>
>> + int op;
>
> The variable name isn't helpful. It's overloaded for pipe index and
> descriptor index in the child. Maybe use three variables with
> appropriate names instead?
Alright, I used a variable for each case as before.
>
>> + {
>> + posix_spawn_file_actions_t fa;
>> + /* posix_spawn_file_actions_init does not fail. */
>> + __posix_spawn_file_actions_init (&fa);
>>
>> - /* If any stream from previous popen() calls has fileno
>> - child_std_end, it has been already closed by the dup2 syscall
>> - above. */
>> - if (fd != child_std_end)
>> - __close_nocancel (fd);
>> - }
>> + /* The descriptor is already in the one the child will use. In this case
>> + it must be moved to another one, otherwise there is no safe way to
>> + remove the close-on-exec flag in the child without creating a FD leak
>> + race in the parent. */
>
> I think “in the one” is a bit confusing here.
I meant 'The descriptor is already the one the child will use', I fixed that.
>
>> + if (pipe_fds[1 - op] == 1 - op)
>> + {
>> + int tmp = __fcntl (1 - op, F_DUPFD_CLOEXEC, 0);
>> + if (tmp < 0)
>> + goto spawn_failure;
>> + __close_nocancel (pipe_fds[1 - op]);
>
> Missing close of pipe_fds[op] in the error case; spawn_failure only
> closes the parent end.
Ack, I have fixed it.
>
>> + if (__posix_spawn_file_actions_adddup2 (&fa, pipe_fds[1 - op], 1 - op)
>> + != 0)
>> + goto spawn_failure;
>
> Likewise, missing close of parent end.
Ack, I have fixed it.
>
>> + /* POSIX.2: "popen() shall ensure that any streams from previous popen()
>> + calls that remain open in the parent process are closed in the new
>> + child process." */
>> +#ifdef _IO_MTSAFE_IO
>> + _IO_cleanup_region_start_noarg (unlock);
>> + _IO_lock_lock (proc_file_chain_lock);
>> +#endif
>
> You can assume that _IO_MTSAFE_IO is always defined here and not carry
> over the preprocessor conditionals.
Could we move this change to future cleanup patch instead? I would like to
add for consistency with other _IO_MTSAFE_IO usage and to avoid remove all
_IO_MTSAFE_IO on this patch specifically.
>
>> + for (struct _IO_proc_file *p = proc_file_chain; p; p = p->next)
>> + {
>> + int fd = _IO_fileno ((FILE *) p);
>> +
>> + /* If any stream from previous popen() calls has fileno
>> + child_send, it has been already closed by the dup2 syscall
>> + above. */
>> + if (fd != 1 - op
>> + && __posix_spawn_file_actions_addclose (&fa, fd) != 0)
>> + goto spawn_failure;
>> + }
>
> The jump out of cleanup region is undefined, I think. Missing close of
> parent end.
Indeed, I changed it.
>
>> +#ifdef _IO_MTSAFE_IO
>> + _IO_lock_unlock (proc_file_chain_lock);
>> + _IO_cleanup_region_end (0);
>> +#endif
>> +
>> + if (__posix_spawn (&((_IO_proc_file *) fp)->pid, _PATH_BSHELL, &fa, 0,
>> + (char *const[]){ (char*) "sh", (char*) "-c",
>> + (char *) command, NULL }, __environ) != 0)
>> + {
>> + spawn_failure:
>> + __posix_spawn_file_actions_destroy (&fa);
>> + __close_nocancel (pipe_fds[1 - op]);
>> + __set_errno (ENOMEM);
>> + return NULL;
>
> Not sure what's supposd to happen to the parent end in this case and if
> you need to close it here.
I think we do need to close both ends otherwise we will leak parent's
descriptor.
>
>> + }
>> +
>> + __posix_spawn_file_actions_destroy (&fa);
>> + }
>> + __close_nocancel (pipe_fds[1 - op]);
>> + if (((_IO_proc_file *) fp)->pid < 0)
>> {
>> - __close_nocancel (parent_end);
>> + __close_nocancel (pipe_fds[op]);
>> return NULL;
>> }
>
> How can ->pid be negative without a posix_spawn failure?
It can not, I have remove it.