/proc/$PID/cmdline: fixup empty ARGV case

/proc/*/cmdline code checks if it should look at ENVP area by checking
last byte of ARGV area:

	rv = access_remote_vm(mm, arg_end - 1, &c, 1, 0);
	if (rv <= 0)
		goto out_free_page;

If ARGV is somehow made empty (by doing execve(..., NULL, ...) or
manually setting ->arg_start and ->arg_end to equal values), the decision
will be based on byte which doesn't even belong to ARGV/ENVP.

So, quickly check if ARGV area is empty and report 0 to match previous
behaviour.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 87782e874b6a..aa50d1ac28fc 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -243,6 +243,11 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
         len1 = arg_end - arg_start;
         len2 = env_end - env_start;
 
+        /* Empty ARGV. */
+        if (len1 == 0) {
+                rv = 0;
+                goto out_free_page;
+        }
         /*
          * Inherently racy -- command line shares address space
          * with code and data.