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gspawn: Implement fd remapping for posix_spawn codepath

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This means that GSubprocess will (sometimes) be able to use the
optimized posix_spawn codepath instead of having to fall back to
fork/exec.
This commit is contained in:
Michael Catanzaro 2021-12-14 13:36:28 -06:00
parent f9780c6bee
commit 7d5bdff6d9
1 changed files with 61 additions and 7 deletions
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68
glib/gspawn.c
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@ -1880,18 +1880,24 @@ do_posix_spawn (const gchar * const *argv,
gint *child_close_fds,
gint stdin_fd,
gint stdout_fd,
gint stderr_fd)
gint stderr_fd,
const gint *source_fds,
const gint *target_fds,
gsize n_fds)
{
pid_t pid;
gint *duped_source_fds = NULL;
gint max_target_fd = 0;
const gchar * const *argv_pass;
posix_spawnattr_t attr;
posix_spawn_file_actions_t file_actions;
gint parent_close_fds[3];
gint num_parent_close_fds = 0;
gsize num_parent_close_fds = 0;
GSList *child_close = NULL;
GSList *elem;
sigset_t mask;
int i, r;
gsize i;
int r;
if (*argv[0] == '\0')
{
@ -2005,6 +2011,46 @@ do_posix_spawn (const gchar * const *argv,
goto out_close_fds;
}
/* If source_fds[i] != target_fds[i], we need to handle the case
* where the user has specified, e.g., 5 -> 4, 4 -> 6. We do this
* by duping the source fds, taking care to ensure the new fds are
* larger than any target fd to avoid introducing new conflicts.
*
* If source_fds[i] == target_fds[i], then we just need to leak
* the fd into the child process, which we *could* do by temporarily
* unsetting CLOEXEC and then setting it again after we spawn if
* it was originally set. POSIX requires that the addup2 action unset
* CLOEXEC if source and target are identical, so you'd think doing it
* manually wouldn't be needed, but unfortunately as of 2021 many
* libcs still don't do so. Example nonconforming libcs:
* Bionic: https://android.googlesource.com/platform/bionic/+/f6e5b582604715729b09db3e36a7aeb8c24b36a4/libc/bionic/spawn.cpp#71
* uclibc-ng: https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/librt/spawn.c?id=7c36bcae09d66bbaa35cbb02253ae0556f42677e#n88
*
* Anyway, unsetting CLOEXEC ourselves would open a small race window
* where the fd could be inherited into a child process if another
* thread spawns something at the same time, because we have not
* called fork() and are multithreaded here. This race is avoidable by
* using dupfd_cloexec, which we already have to do to handle the
* source_fds[i] != target_fds[i] case. So let's always do it!
*/
for (i = 0; i < n_fds; i++)
max_target_fd = MAX (max_target_fd, target_fds[i]);
if (max_target_fd == G_MAXINT)
goto out_close_fds;
duped_source_fds = g_new (gint, n_fds);
for (i = 0; i < n_fds; i++)
duped_source_fds[i] = dupfd_cloexec (source_fds[i], max_target_fd + 1);
for (i = 0; i < n_fds; i++)
{
r = posix_spawn_file_actions_adddup2 (&file_actions, duped_source_fds[i], target_fds[i]);
if (r != 0)
goto out_close_fds;
}
/* Intentionally close the fds in the child as the last file action,
* having been careful not to add the same fd to this list twice.
*
@ -2037,6 +2083,13 @@ out_close_fds:
for (i = 0; i < num_parent_close_fds; i++)
close_and_invalidate (&parent_close_fds [i]);
if (duped_source_fds != NULL)
{
for (i = 0; i < n_fds; i++)
close_and_invalidate (&duped_source_fds[i]);
g_free (duped_source_fds);
}
posix_spawn_file_actions_destroy (&file_actions);
out_free_spawnattr:
posix_spawnattr_destroy (&attr);
@ -2124,10 +2177,8 @@ fork_exec (gboolean intermediate_child,
child_close_fds[n_child_close_fds++] = -1;
#ifdef POSIX_SPAWN_AVAILABLE
/* FIXME: Handle @source_fds and @target_fds in do_posix_spawn() using the
* file actions API. */
if (!intermediate_child && working_directory == NULL && !close_descriptors &&
!search_path_from_envp && child_setup == NULL && n_fds == 0)
!search_path_from_envp && child_setup == NULL)
{
g_trace_mark (G_TRACE_CURRENT_TIME, 0,
"GLib", "posix_spawn",
@ -2144,7 +2195,10 @@ fork_exec (gboolean intermediate_child,
child_close_fds,
stdin_fd,
stdout_fd,
stderr_fd);
stderr_fd,
source_fds,
target_fds,
n_fds);
if (status == 0)
goto success;