[PATCH] posix: Fix return value of system if shell can not be executed [BZ #27053]

[Adhemerval Zanella]

POSIX states that system returned code for failure to execute the shell
shall be as if the shell had terminated using _exit(127).  This
behaviour was removed with 5fb7fc96350575.

This issue should not be possible for pclose because if the shell can
not be executed, popen would not return a valid FILE object.  Other
process execution failures (such as killed by a signal) would be
returned through waitpid from pclose call.

The new tst-system has an underlying issue once new containers tests
are added that might issue the minimal container shell, since it
changes the shell execution mode.  It is not an issue now, since it is
the only container tests for stdlib subforder, and I would like to
avoid setting non parallel execution.  Another possibility if
concurrent container tests issues the minimum shell would be to
add a advisory lock.

Checked on x86_64-linux-gnu.
---
 stdlib/tst-system.c    | 17 +++++++++++++++++
 support/Makefile       |  1 +
 support/xchmod.c       | 30 ++++++++++++++++++++++++++++++
 support/xunistd.h      |  1 +
 sysdeps/posix/system.c |  4 ++++
 5 files changed, 53 insertions(+)
 create mode 100644 support/xchmod.c

diff --git a/stdlib/tst-system.c b/stdlib/tst-system.c
index eddea33f4c..f477aeb267 100644
--- a/stdlib/tst-system.c
+++ b/stdlib/tst-system.c
@@ -26,6 +26,7 @@
 #include <support/check.h>
 #include <support/temp_file.h>
 #include <support/support.h>
+#include <support/xunistd.h>
 
 static char *tmpdir;
 static long int namemax;
@@ -138,6 +139,22 @@ do_test (void)
     support_capture_subprocess_check (&result, "system", 0, sc_allow_none);
   }
 
+  {
+    struct stat64 st;
+    xstat (_PATH_BSHELL, &st);
+    mode_t mode = st.st_mode;
+    xchmod (_PATH_BSHELL, mode & ~(S_IXUSR | S_IXGRP | S_IXOTH));
+
+    struct support_capture_subprocess result;
+    result = support_capture_subprocess (call_system,
+					 &(struct args) {
+					   "exit 1", 127, 0
+					 });
+    support_capture_subprocess_check (&result, "system", 0, sc_allow_none);
+
+    xchmod (_PATH_BSHELL, st.st_mode);
+  }
+
   TEST_COMPARE (system (""), 0);
 
   return 0;
diff --git a/support/Makefile b/support/Makefile
index f5f59bf8d2..c89b16bbcb 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -90,6 +90,7 @@ libsupport-routines = \
   xchroot \
   xclock_gettime \
   xclose \
+  xchmod \
   xconnect \
   xcopy_file_range \
   xdlfcn \
diff --git a/support/xchmod.c b/support/xchmod.c
new file mode 100644
index 0000000000..5e403c7cc2
--- /dev/null
+++ b/support/xchmod.c
@@ -0,0 +1,30 @@
+/* chmod with error checking.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/xunistd.h>
+#include <support/check.h>
+
+#include <sys/stat.h>
+
+void
+xchmod (const char *pathname, mode_t mode)
+{
+  int r = chmod (pathname, mode);
+  if (r < 0)
+    FAIL_EXIT1 ("chmod (%s, %d): %m", pathname, mode);
+}
diff --git a/support/xunistd.h b/support/xunistd.h
index b299db77ba..29b9a028b0 100644
--- a/support/xunistd.h
+++ b/support/xunistd.h
@@ -45,6 +45,7 @@ long xsysconf (int name);
 long long xlseek (int fd, long long offset, int whence);
 void xftruncate (int fd, long long length);
 void xsymlink (const char *target, const char *linkpath);
+void xchmod (const char *pathname, mode_t mode);
 
 /* Equivalent of "mkdir -p".  */
 void xmkdirp (const char *, mode_t);
diff --git a/sysdeps/posix/system.c b/sysdeps/posix/system.c
index a03f478fc7..b86fc9b756 100644
--- a/sysdeps/posix/system.c
+++ b/sysdeps/posix/system.c
@@ -175,6 +175,10 @@ do_system (const char *line)
       __libc_cleanup_region_end (0);
 #endif
     }
+  else
+   /* POSIX states that failure to execute the shell should return
+      as if the shell had terminated using _exit(127).  */
+   status = W_EXITCODE (127, 0);
 
   DO_LOCK ();
   if (SUB_REF () == 0)
-- 
2.25.1