mmap: fix petty bug in anonymous shared mmap offset handling

Anonymous mappings should ignore offset but shared anonymous mapping
forgot to clear it and makes the following legit test program trigger
SIGBUS.

 #include <sys/mman.h>
 #include <stdio.h>
 #include <errno.h>

 #define PAGE_SIZE	4096

 int main(void)
 {
	 char *p;
	 int i;

	 p = mmap(NULL, 2 * PAGE_SIZE, PROT_READ|PROT_WRITE,
		  MAP_SHARED|MAP_ANONYMOUS, -1, PAGE_SIZE);
	 if (p == MAP_FAILED) {
		 perror("mmap");
		 return 1;
	 }

	 for (i = 0; i < 2; i++) {
		 printf("page %d\n", i);
		 p[i * 4096] = i;
	 }
	 return 0;
 }

Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Hugh Dickins <hugh@veritas.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 339cf5c4d5d8..e7a5a68a9c2e 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1030,6 +1030,10 @@ unsigned long do_mmap_pgoff(struct file * file, unsigned long addr,
 	} else {
 		switch (flags & MAP_TYPE) {
 		case MAP_SHARED:
+			/*
+			 * Ignore pgoff.
+			 */
+			pgoff = 0;
 			vm_flags |= VM_SHARED | VM_MAYSHARE;
 			break;
 		case MAP_PRIVATE: