[PATCH] Limit sendfile() to 2^31-PAGE_CACHE_SIZE bytes without error

["H. Peter Anvin" <hpa@zytor.com>]

sendfile() has a limit of 2^31-1 bytes even on 64-bit platforms.  Linus 
wants to maintain it in order to avoid potential future security bugs 
(always a good thing.)

This patch changes the behaviour from returning EINVAL when this limit 
is exceeded to returning a short count.  This means that a 
properly-written userspace program will simply loop around and continue; 
 it will expose bugs in improperly-written userspace programs, which is 
also a good thing.  Additionally, the limit becomes an issue that is 
completely contained within the kernel, and not encoded in the kernel 
ABI, so it can be changed in the future.

(I set the limit to 2^31-PAGE_CACHE_SIZE so that a transfer that starts 
at the beginning of the file will continue to be page-aligned.)

The

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/fs/read_write.c b/fs/read_write.c
index a091ee4..3712886 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -9,6 +9,7 @@
 #include <linux/fcntl.h>
 #include <linux/file.h>
 #include <linux/uio.h>
+#include <linux/pagemap.h>
 #include <linux/smp_lock.h>
 #include <linux/fsnotify.h>
 #include <linux/security.h>
@@ -631,6 +632,9 @@ static ssize_t do_sendfile(int out_fd, i
 	ssize_t retval;
 	int fput_needed_in, fput_needed_out;
 
+	/* Avoid potential security holes.  User space will get a short count and should loop. */
+	count = min(count, (size_t)0x80000000-PAGE_CACHE_SIZE);
+
 	/*
 	 * Get input file, and verify that it is ok..
 	 */