[PATCH v2] linux: Fix __closefrom_fallback iterates until max int (BZ#28993)

[Adhemerval Zanella]

The __closefrom_fallback tries to get a available file descriptor
if the initial open ("/proc/self/fd/", ...) fails.  It assumes the
failure would be only if procfs is not mount (ENOENT), however if
the the proc file is not accessible (due some other kernel filtering
such apparmor) it will iterate over a potentially large file set
issuing close calls.

It should only try the close fallback if open returns EMFILE.

Checked on x86_64-linux-gnu.
---
v2: Fix wrong call of __close_nocancel if case of failure.
---
 sysdeps/unix/sysv/linux/closefrom_fallback.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/closefrom_fallback.c b/sysdeps/unix/sysv/linux/closefrom_fallback.c
index 60101aa3ba..da1f29752d 100644
--- a/sysdeps/unix/sysv/linux/closefrom_fallback.c
+++ b/sysdeps/unix/sysv/linux/closefrom_fallback.c
@@ -30,15 +30,13 @@
 _Bool
 __closefrom_fallback (int from, _Bool dirfd_fallback)
 {
-  bool ret = false;
-
   int dirfd = __open_nocancel (FD_TO_FILENAME_PREFIX, O_RDONLY | O_DIRECTORY,
                                0);
   if (dirfd == -1)
     {
       /* The closefrom should work even when process can't open new files.  */
-      if (errno == ENOENT || !dirfd_fallback)
-        goto err;
+      if (errno != EMFILE || !dirfd_fallback)
+	return false;
 
       for (int i = from; i < INT_MAX; i++)
         {
@@ -54,6 +52,7 @@ __closefrom_fallback (int from, _Bool dirfd_fallback)
     }
 
   char buffer[1024];
+  bool ret = false;
   while (true)
     {
       ssize_t ret = __getdents64 (dirfd, buffer, sizeof (buffer));
-- 
2.32.0