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Differential D19058 Diff 62099 lib/libc/gen/posix_spawn.c

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lib/libc/gen/posix_spawn.c

Show First 20 Lines • Show All 188 Lines • ▼ Show 20 Linesprocess_file_actions(const posix_spawn_file_actions_t fa)
STAILQ_FOREACH(fae, &fa->fa_list, fae_list) { STAILQ_FOREACH(fae, &fa->fa_list, fae_list) {
error = process_file_actions_entry(fae); error = process_file_actions_entry(fae);
if (error) if (error)
return (error); return (error);
} }
return (0); return (0);
} }
struct posix_spawn_args {
const char *path;
const posix_spawn_file_actions_t *fa;
const posix_spawnattr_t *sa;
char * const * argv;
char * const * envp;
int use_env_path;
int error;
};
#if defined(__i386__) || defined(__amd64__)
#define _RFORK_THREAD_STACK_SIZE 4096
#endif
kibUnsubmitted
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You do not need both defines.

On amd64, there is so called red zone, an area in the stack below %rsp that compiler is free to use. Red zone size is 128 bytes, which leaves 192 - 128 = 64 bytes to use until formal overflow. IMO it is too optimistic.

kib: You do not need both defines. On amd64, there is so called red zone, an area in the stack…
kevansAuthorUnsubmitted
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Ah, that's good to know- 192 was a rough approximation based on measured-once...

do_posix_spawn -> process_spawnattr -> (syscalls)/sigismember

do_posix_spawn -> process_file_actions -> process_file_actions_entry -> (syscalls)

So accounting for 3/4-deep, perhaps 768 as a conservative estimate? Even 1k wouldn't exactly hurt anything, that's 'small potatoes' as far as posix_spawn memory requirements go, I would think.

kevans: Ah, that's good to know- 192 was a rough approximation based on measured-once...
jillesUnsubmitted
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Various exported functions such as _execve() are called, which might cause rtld and/or libthr to be involved and stack usage to go up greatly. In principle, it's possible to do the necessary work with only unexported aliases and/or do preparations like getenv("PATH") in the parent process.

jilles: Various exported functions such as _execve() are called, which might cause rtld and/or libthr…
kevansAuthorUnsubmitted
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Hmm... at least most of this should be successfully avoiding libthr, as it's "in an undefined state after the vfork" according to it.

What specifically would you propose here? Switching _execve over to the unwrapped version and using an even more conservative 1k/2k/4k for the stack size to catch any edge cases?

kevans: Hmm... at least most of this should be successfully avoiding libthr, as it's "in an undefined…
kevansAuthorUnsubmitted
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Alternatively, how bad of an idea would it be to fetch the stack pointer, subtract past the red zone, and use that for the rfork_thread stack? This is only x86 that has this problem, so it's really tempting.

kevans: Alternatively, how bad of an idea would it be to fetch the stack pointer, subtract past the red…
kibUnsubmitted
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I would go forward with 2K or even 4K stack and claim that the issue is resolved.

kib: I would go forward with 2K or even 4K stack and claim that the issue is resolved.
jillesUnsubmitted
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What could be a problem is if the child is killed forcibly while holding a lock, since this would likely cause the parent to deadlock later. This could happen a signal is sent to the process group for which the parent has a handler that does little and the default action is to terminate the process. (Getting this to happen with SIGKILL would be very unlikely since it is hard to send a signal to the child process only.) With rtld, this can happen only with the first posix_spawn call using particular functionality in the process, and libthr seems to be successfully avoided.

This seems rare enough not to block this review on it.

jilles: What could be a problem is if the child is killed forcibly while holding a lock, since this…
static int static int
_posix_spawn_thr(void *data)
{
struct posix_spawn_args *psa;
char * const *envp;
psa = data;
if (psa->sa != NULL) {
psa->error = process_spawnattr(*psa->sa);
if (psa->error)
_exit(127);
}
if (psa->fa != NULL) {
psa->error = process_file_actions(*psa->fa);
if (psa->error)
_exit(127);
}
envp = psa->envp != NULL ? psa->envp : environ;
if (psa->use_env_path)
_execvpe(psa->path, psa->argv, envp);
else
_execve(psa->path, psa->argv, envp);
psa->error = errno;
/* This is called in such a way that it must not exit. */
jillesUnsubmitted
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Do you mean "must not return"?

jilles: Do you mean "must not return"?
kevansAuthorUnsubmitted
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Hmmm... I caught that once, but it looks like I had only fixed it locally in the diff that I uploaded (61840)... fixed for next update.

kevans: Hmmm... I caught that once, but it looks like I had only fixed it locally in the diff that I…
_exit(127);
}
static int
do_posix_spawn(pid_t *pid, const char *path, do_posix_spawn(pid_t *pid, const char *path,
const posix_spawn_file_actions_t *fa, const posix_spawn_file_actions_t *fa,
const posix_spawnattr_t *sa, const posix_spawnattr_t *sa,
char * const argv[], char * const envp[], int use_env_path) char * const argv[], char * const envp[], int use_env_path)
{ {
struct posix_spawn_args psa;
pid_t p; pid_t p;
volatile int error = 0; #ifdef _RFORK_THREAD_STACK_SIZE
char *stack;
stack = malloc(_RFORK_THREAD_STACK_SIZE);
if (stack == NULL)
return (ENOMEM);
#endif
psa.path = path;
psa.fa = fa;
psa.sa = sa;
psa.argv = argv;
psa.envp = envp;
psa.use_env_path = use_env_path;
psa.error = 0;
/*
kibUnsubmitted
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It might be wise to fall back to vfork() on EINVAL.

kib: It might be wise to fall back to vfork() on EINVAL.
* Passing RFSPAWN to rfork(2) gives us effectively a vfork that drops
* non-ignored signal handlers. We'll fall back to the slightly less
kibUnsubmitted
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Perhaps explain that this happens on old kernels.

kib: Perhaps explain that this happens on old kernels.
* ideal vfork(2) if we get an EINVAL from rfork -- this should only
* happen with newer libc on older kernel that doesn't accept
jillesUnsubmitted
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I'm surprised that this works at all on (among others) amd64: the child pops the return address and overwrites it with new return addresses for calls it makes; how can the parent's return (the ret instruction in rfork.S from SYS.h) go to the right place? Note that vfork has a hand-written stub that deals with this problem.

On architectures such as sparc64, arm and mips where subroutine return addresses are stored in a register and system call stubs do not use the stack, this should not be an issue.

jilles: I'm surprised that this works at all on (among others) amd64: the child pops the return address…
kibUnsubmitted
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Indeed, perhaps rfork_thread(3) should be used. I hope that it still works.

kib: Indeed, perhaps rfork_thread(3) should be used. I hope that it still works.
kevansAuthorUnsubmitted
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I'll double check my test system and rework this

kevans: I'll double check my test system and rework this
jillesUnsubmitted
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Using rfork_thread(3) to implement something vfork-like does not seem a violation of its deprecation. The deprecation is about using rfork_thread(3) to create threads.

rfork_thread(3)'s API is deficient in specifying the stack: it only takes an initial stack pointer, where I would expect a base and a size. Perhaps because of that or because it is deprecated, it is only available on i386 and amd64.

Using a separate stack for the child process does avoid GCC bugs like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21161.

jilles: Using rfork_thread(3) to implement something vfork-like does not seem a violation of its…
kibUnsubmitted
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By 'hope' I mean that we do not corrupt userspace registers in the syscall for rfork_thread() to work. Note that fast syscall on amd64 does not restore all registers as an optimization.

Fortunately, child return from fork sets PCB_FULL_IRET in cpu_fork(), so all registers are restored. From my reading of the rfork_thread() code, it does not matter for parent.

But I did not verified it by a test.

kib: By 'hope' I mean that we do not corrupt userspace registers in the syscall for rfork_thread()…
jillesUnsubmitted
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rfork_thread(RFPROC | RFFDG | RFMEM, ...) appears to work still (on stable/11 amd64 and head amd64).

rfork_thread.S only depends on %rsi being preserved in the child, but vfork.S depends on %rsi being preserved in both parent and child. The fast return path of fast_syscall additionally preserves %rdi and %rsi, calling that a bonus in comments. I don't think it is a bonus -- vfork.S depends on it and has no good alternative location to store the parent's return address (thread local storage? yuck).

jilles: rfork_thread(RFPROC | RFFDG | RFMEM, ...) appears to work still (on stable/11 amd64 and head…
kibUnsubmitted
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From my reading, the child dependencies are %rsi, %r12, and %rbx. But they are handler by iret path.

kib: From my reading, the child dependencies are %rsi, %r12, and %rbx. But they are handler by iret…
* RFSPAWN.
*/
#ifdef _RFORK_THREAD_STACK_SIZE
jillesUnsubmitted
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In a threaded process, this will call libthr's __thr_sigprocmask which leaves SIGCANCEL/SIGTHR unmasked. Perhaps this should be __sigprocmask which is an unexported weak alias for __sys_sigprocmask.

jilles: In a threaded process, this will call libthr's `__thr_sigprocmask` which leaves…
/*
* x86 stores the return address on the stack, so rfork(2) cannot work
* as-is because the child would clobber the return address om the
* parent. Because of this, we must use rfork_thread instead while
* almost every other arch stores the return address in a register.
*/
p = rfork_thread(RFSPAWN, stack + _RFORK_THREAD_STACK_SIZE,
_posix_spawn_thr, &psa);
free(stack);
#else
p = rfork(RFSPAWN);
if (p == 0)
/* _posix_spawn_thr does not return */
_posix_spawn_thr(&psa);
#endif
/*
* The above block should leave us in a state where we've either
* succeeded and we're ready to process the results, or we need to
* fallback to vfork() if the kernel didn't like RFSPAWN.
*/
if (p == -1 && errno == EINVAL) {
p = vfork(); p = vfork();
switch (p) { if (p == 0)
case -1: /* _posix_spawn_thr does not return */
return (errno); _posix_spawn_thr(&psa);
case 0:
if (sa != NULL) {
error = process_spawnattr(*sa);
if (error)
_exit(127);
} }
if (fa != NULL) { if (p == -1)
error = process_file_actions(*fa); return (errno);
if (error) if (psa.error != 0)
_exit(127); /* Failed; ready to reap */
}
if (use_env_path)
_execvpe(path, argv, envp != NULL ? envp : environ);
else
_execve(path, argv, envp != NULL ? envp : environ);
error = errno;
_exit(127);
default:
if (error != 0)
_waitpid(p, NULL, WNOHANG); _waitpid(p, NULL, WNOHANG);
else if (pid != NULL) else if (pid != NULL)
/* exec succeeded */
*pid = p; *pid = p;
return (error); return (psa.error);
}
} }
int int
posix_spawn(pid_t *pid, const char *path, posix_spawn(pid_t *pid, const char *path,
const posix_spawn_file_actions_t *fa, const posix_spawn_file_actions_t *fa,
const posix_spawnattr_t *sa, const posix_spawnattr_t *sa,
char * const argv[], char * const envp[]) char * const argv[], char * const envp[])
{ {
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