Hi,
Currently, if i mmap() a file PROT_WRITE only and then first read from it
and then write to it, i get a SEGV. However, if i write to it first and
then read from it, i get no SEGV. This seems rather inconsistent.
The current implementation seems to be to make PROT_WRITE imply PROT_READ,
however it does not quite work correctly. The patch below resolves this
issue, by explicitly setting the PROT_READ flag when PROT_WRITE is
requested.
This might appear at first as a possible permissions subversion, as i
could get PROT_READ on a file that i only have write permission
to...however, the mmap implementation requires that the file be opened
with at least read access already. Thus, i don't believe there is any
issue with regards to permissions.
Another consequenece of this patch is that it forces PROT_READ even for
architectures that might be able to support it, (I know that x86, x86_64
and ia64 do not) but i think this is best for portability.
This was originally reported by Doug Chapman.
thanks,
-Jason
Signed-off-by: Jason Baron <[email protected]>
--- linux-2.6/mm/mmap.c.bak 2006-06-21 17:07:52.000000000 -0400
+++ linux-2.6/mm/mmap.c 2006-06-21 17:22:54.000000000 -0400
@@ -910,6 +910,13 @@
if (!(file && (file->f_vfsmnt->mnt_flags & MNT_NOEXEC)))
prot |= PROT_EXEC;
+ /* SuSv3: "if the application requests only PROT_WRITE, the
+ * implementation may also allow read access."
+ */
+
+ if (prot & PROT_WRITE)
+ prot |= PROT_READ;
+
if (!len)
return -EINVAL;
-
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