Re: [patch] mmap bug with drivers that adjust vm_start

Benjamin LaHaise (
Tue, 26 Mar 2002 15:43:45 -0500

On Tue, Mar 26, 2002 at 08:15:02PM +0100, Andrea Arcangeli wrote:
> > we could BUG() on getting a vma back from the new find_vma_prepare call.
> yes, it sounds a good idea to verify there's no other mapping in the way
> of the relocation (until a better fix is implemented), it's a slow path
> so we won't hurt performance.

Okay, updated patch for this is below. I also added a printk to give us
which mmap operation was invoked to aid in debugging.

> The good reason, is that currently we're literally corrupting the
> userspace with the senseless do_munmap call in the add<->addr+len area
> before the ->mmap lowlevel callback. And such an munmap is certainly not
> required to maintain source and binary compatibility (otherwise it would
> be insane in the first place :).

Ah, right. I disallow MAP_FIXED addresses that are not the "correct" offset,
so this case would fail, albeit with the do_munmap occurring. Personally,
I would rather see an mmap fail if it would collide with an existing mapping,
but that might break some applications.

Thanks for looking this over. Cheers,


"A man with a bass just walked in,
 and he's putting it down
 on the floor."

:r ~/patches/v2.4.19-pre4-mmap_fix-2.diff --- retest.3/mm/ Mon Mar 25 19:38:10 2002 +++ retest.3/mm/mmap.c Tue Mar 26 15:01:47 2002 @@ -14,6 +14,7 @@ #include <linux/file.h> #include <linux/fs.h> #include <linux/personality.h> +#include <linux/compiler.h> #include <asm/uaccess.h> #include <asm/pgalloc.h> @@ -548,7 +549,19 @@ * Answer: Yes, several device drivers can do it in their * f_op->mmap method. -DaveM */ - addr = vma->vm_start; + if (addr != vma->vm_start) { + /* Since addr changed, we rely on the mmap op to prevent + * collisions with existing vmas and just use find_vma_prepare + * to update the tree pointers. + */ + addr = vma->vm_start; + if (unlikely(NULL != find_vma_prepare(mm, addr, &prev, + &rb_link, &rb_parent))) { + printk(KERN_ERR "buggy mmap operation: [<%p>]\n", + file ? file->f_op->mmap : NULL); + BUG(); + } + } vma_link(mm, vma, prev, rb_link, rb_parent); if (correct_wcount) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to More majordomo info at Please read the FAQ at