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SubjectRe: mmap bug?
On Fri, 29 May 1998, Natarajan Krishnaswami wrote:

> [someone] wrote:
> >> It seems that attempts to mmap a region of size zero causes
> >> mmap to return zero. Is this a bug?
> >>
> >> Since it doesn't return -1, it looks like success, but of
> >> course dereferencing this NULL pointer causes a crash.
>
> Zero is a multiple of the page size, so there is no partial page mapped:
> the map is empty. Returning a pointer which may not be dereferenced
> seems reasonable. However, mmap is explicitly forbidden from returning
> 0, and SIGBUS would probably be the correct signal to deliver if an
> address in a 0-length map were dereferenced.

Thank you.

I was surprised at the number of people prepared to tell me not to
mmap zero length areas. Here's a trivial patch to fix this, although
I don't know if this will affect many programs.


--- mm/mmap.c.old Fri May 29 19:48:18 1998
+++ mm/mmap.c Fri May 29 19:48:45 1998
@@ -165,7 +165,7 @@
int correct_wcount = 0, error;

if ((len = PAGE_ALIGN(len)) == 0)
- return addr;
+ return -ENXIO;

if (len > TASK_SIZE || addr > TASK_SIZE-len)
return -EINVAL;
Cheers,
Matthew.


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