From fe05e1cb6d64dba6172249c79526f1e9af8f2bfd Mon Sep 17 00:00:00 2001

From: Adhemerval Zanella <adhemerval.zanella@linaro.org>

Date: Thu, 12 Oct 2017 15:20:57 -0300

Subject: [PATCH] posix: Fix improper assert in Linux posix_spawn (BZ#22273)

As noted by Florian Weimer, current Linux posix_spawn implementation

can trigger an assert if the auxiliary process is terminated before

actually setting the err member:

340 /* Child must set args.err to something non-negative - we rely on

341 the parent and child sharing VM. */

342 args.err = -1;

[...]

362 new_pid = CLONE (__spawni_child, STACK (stack, stack_size), stack_size,

363 CLONE_VM | CLONE_VFORK | SIGCHLD, &args);

364

365 if (new_pid > 0)

366 {

367 ec = args.err;

368 assert (ec >= 0);

Another possible issue is killing the child between setting the err and

actually calling execve. In this case the process will not ran, but

posix_spawn also will not report any error:

269

270 args->err = 0;

271 args->exec (args->file, args->argv, args->envp);

As suggested by Andreas Schwab, this patch removes the faulty assert

and also handles any signal that happens before fork and execve as the

spawn was successful (and thus relaying the handling to the caller to

figure this out). Different than Florian, I can not see why using

atomics to set err would help here, essentially the code runs

sequentially (due CLONE_VFORK) and I think it would not be legal the

compiler evaluate ec without checking for new_pid result (thus there

is no need to compiler barrier).

Summarizing the possible scenarios on posix_spawn execution, we

have:

1. For default case with a success execution, args.err will be 0, pid

will not be collected and it will be reported to caller.

2. For default failure case, args.err will be positive and the it will

be collected by the waitpid. An error will be reported to the

caller.

3. For the unlikely case where the process was terminated and not

collected by a caller signal handler, it will be reported as succeful

execution and not be collected by posix_spawn (since args.err will

be 0). The caller will need to actually handle this case.

4. For the unlikely case where the process was terminated and collected

by caller we have 3 other possible scenarios:

4.1. The auxiliary process was terminated with args.err equal to 0:

it will handled as 1. (so it does not matter if we hit the pid

reuse race since we won't possible collect an unexpected

process).

4.2. The auxiliary process was terminated after execve (due a failure

in calling it) and before setting args.err to -1: it will also

be handle as 1. but with the issue of not be able to report the

caller a possible execve failures.

4.3. The auxiliary process was terminated after args.err is set to -1:

this is the case where it will be possible to hit the pid reuse

case where we will need to collected the auxiliary pid but we

can not be sure if it will be expected one. I think for this

case we need to actually change waitpid to use WNOHANG to avoid

hanging indefinitely on the call and report an error to caller

since we can't differentiate between a default failure as 2.

and a possible pid reuse race issue.

Checked on x86_64-linux-gnu.

* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Handle the case where

the auxiliary process is terminated by a signal before calling _exit

or execve.

diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c

index dea1650..d15fbb1 100644

--- a/sysdeps/unix/sysv/linux/spawni.c

+++ b/sysdeps/unix/sysv/linux/spawni.c

@@ -17,7 +17,6 @@

<http://www.gnu.org/licenses/>. */

#include <spawn.h>

-#include <assert.h>

#include <fcntl.h>

#include <paths.h>

#include <string.h>

@@ -268,7 +267,6 @@ __spawni_child (void *arguments)

__sigprocmask (SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)

? &attr->__ss : &args->oldmask, 0);

- args->err = 0;

args->exec (args->file, args->argv, args->envp);

/* This is compatibility function required to enable posix_spawn run

@@ -339,7 +337,7 @@ __spawnix (pid_t * pid, const char *file,

/* Child must set args.err to something non-negative - we rely on

the parent and child sharing VM. */

- args.err = -1;

+ args.err = 0;

args.file = file;

args.exec = exec;

args.fa = file_actions;

@@ -362,12 +360,26 @@ __spawnix (pid_t * pid, const char *file,

new_pid = CLONE (__spawni_child, STACK (stack, stack_size), stack_size,

CLONE_VM | CLONE_VFORK | SIGCHLD, &args);

+ /* It needs to collect the case where the auxiliary process was created

+ but failed to execute the file (due either any preparation step or

+ for execve itself). */

if (new_pid > 0)

{

+ /* Also, it handles the unlikely case where the auxiliary process was

+ terminated before calling execve as if it was successfully. The

+ args.err is set to 0 as default and changed to a positive value

+ only in case of failure, so in case of premature termination

+ due a signal args.err will remain zeroed and it will be up to

+ caller to actually collect it. */

ec = args.err;

- assert (ec >= 0);

- if (ec != 0)

- __waitpid (new_pid, NULL, 0);

+ if (ec > 0)

+ /* There still an unlikely case where the child is cancelled after

+ setting args.err, due to a positive error value. Also due a

+ possible pid reuse race (where the kernel allocated the same pid

+ to unrelated process) we need not to undefinitely hang expecting

+ an invalid pid. In both cases an error is returned to the

+ caller. */

+ __waitpid (new_pid, NULL, WNOHANG);

}

else

ec = -new_pid;

--

2.9.3