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SubjectRe: [PATCH #2.6.27-rc5] mmap: fix petty bug in anonymous shared mmap offset handling
On Wed, 3 Sep 2008, Tejun Heo wrote:

> 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>

Fair enough. The current behaviour has (almost) never bothered us,
so I'm uncertain if your test is legit, but I can't see any reason
to object to the change. Particularly since (just out of sight below
the context of your patch) we force pgoff in the MAP_PRIVATE case.

Acked-by: Hugh Dickins <hugh@veritas.com>

> ---
> mm/mmap.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 339cf5c..e7a5a68 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:


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