linux: sysconf: limit _SC_MAX_ARG to 6 MiB (BZ #25305)

Since Linux 4.13, kernel limits the maximum command line arguments
length to 6 MiB [1].  Normally the limit is still quarter of the maximum
stack size but if that limit exceeds 6 MiB it's clamped down.

glibc's __sysconf implementation for Linux platform is not aware of
this limitation and for stack sizes of over 24 MiB it returns higher
ARG_MAX than Linux will actually accept.  This can be verified by
executing the following application on Linux 4.13 or newer:

    #include <stdio.h>
    #include <string.h>
    #include <sys/resource.h>
    #include <sys/time.h>
    #include <unistd.h>

    int main(void) {
            const struct rlimit rlim = { 40 * 1024 * 1024,
                                         40 * 1024 * 1024 };
            if (setrlimit(RLIMIT_STACK, &rlim) < 0) {
                    perror("setrlimit: RLIMIT_STACK");
                    return 1;
            }

            printf("ARG_MAX     : %8ld\n", sysconf(_SC_ARG_MAX));
            printf("63 * 100 KiB: %8ld\n", 63L * 100 * 1024);
            printf("6 MiB       : %8ld\n", 6L * 1024 * 1024);

            char str[100 * 1024], *argv[64], *envp[1];
            memset(&str, 'A', sizeof str);
            str[sizeof str - 1] = '\0';
            for (size_t i = 0; i < sizeof argv / sizeof *argv - 1; ++i) {
                    argv[i] = str;
            }
            argv[sizeof argv / sizeof *argv - 1] = envp[0] = 0;

            execve("/bin/true", argv, envp);
            perror("execve");
            return 1;
    }

On affected systems the program will report ARG_MAX as 10 MiB but
despite that executing /bin/true with a bit over 6 MiB of command line
arguments will fail with E2BIG error.  Expected result is that ARG_MAX
is reported as 6 MiB.

Update the __sysconf function to clamp ARG_MAX value to 6 MiB if it
would otherwise exceed it.  This resolves bug #25305 which was market
WONTFIX as suggested solution was to cap ARG_MAX at 128 KiB.

As an aside and point of comparison, bionic (a libc implementation for
Android systems) decided to resolve this issue by always returning 128
KiB ignoring any potential xargs regressions [2].

On older kernels this results in returning overly conservative value
but that's a safer option than being aggressive and returning invalid
value on recent systems.  It's also worth noting that at this point
all supported Linux releases have the 6 MiB barrier so only someone
running an unsupported kernel version would get incorrectly truncated
result.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

[1] See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=da029c11e6b12f321f36dac8771e833b65cec962
[2] See baed51ee3a

sysdeps/unix/sysv/linux/sysconf.c

@@ -33,6 +33,9 @@
    actual value varies based on the stack size.  */
 #define legacy_ARG_MAX 131072
 
+/* Newer kernels (4.13) limit the maximum command line arguments lengths to
+   6MiB.  */
+#define maximum_ARG_MAX 6291456
 
 static long int posix_sysconf (int name);
 
@@ -55,7 +58,10 @@ __sysconf (int name)
         struct rlimit rlimit;
         /* Use getrlimit to get the stack limit.  */
         if (__getrlimit (RLIMIT_STACK, &rlimit) == 0)
-	  return MAX (legacy_ARG_MAX, rlimit.rlim_cur / 4);
+	  {
+	    const long int limit = MAX (legacy_ARG_MAX, rlimit.rlim_cur / 4);
+	    return MIN (limit, maximum_ARG_MAX);
+	  }
 
         return legacy_ARG_MAX;
       }