Re: [PATCH?] Crash in 2.4.17/ptrace

Andrew Morton (akpm@zip.com.au)
Mon, 28 Jan 2002 13:03:05 -0800


Daniel Jacobowitz wrote:
>
> I've been debugging frame buffer graphics lately, and encountering a
> very annoying problem. If the debugee has /dev/fb/0 mapped, and I try
> to print out the contents of a pointer into that buffer, GDB crashes in
> kernel/ptrace.c:access_process_vm. The problem seems to be that
> get_user_pages returns a NULL page. Something as simple as this
> prevents the crash:
>
> --- 2.4.18-pre7/2.4.18-pre7/kernel/ptrace.c Fri Dec 21 12:42:04 2001
> +++ 2.4.17/kernel-source-2.4.17/kernel/ptrace.c Mon Jan 28 15:30:39 2002
> @@ -160,6 +160,18 @@ int access_process_vm(struct task_struct
>
> flush_cache_page(vma, addr);
>
> +#if 1
> + if (!page)
> + {
> + /* FIXME: Writes? */
> + if (!write) memset (buf, 0, bytes);
> + len -= bytes;
> + buf += bytes;
> + continue;
> + }
> +#endif
> +
> +
> maddr = kmap(page);
> if (write) {
> memcpy(maddr + offset, buf, bytes);

Oh nice. And it seems that, say, an O_DIRECT write of, say,
a mmaped framebuffer will also oops the kernel.

Most callers of get_user_pages() aren't prepared for a
null page* in the returned array.

This patch *may* be sufficient, but perhaps get_user_pages()
should just bale out as soon as it finds an invalid page, rather
than sticking a null page * into the returned array and continuing.

--- linux-2.4.18-pre7/mm/memory.c Fri Dec 21 11:19:23 2001
+++ linux-akpm/mm/memory.c Mon Jan 28 12:54:40 2002
@@ -453,6 +453,7 @@ int get_user_pages(struct task_struct *t
vma = find_extend_vma(mm, start);

if ( !vma ||
+ (vma->vm_flags & VM_IO) ||
(!force &&
((write && (!(vma->vm_flags & VM_WRITE))) ||
(!write && (!(vma->vm_flags & VM_READ))) ) )) {

> Of course, I would much rather be able to see the contents of the
> framebuffer. Any suggestions?

Not with this patch, I'm afraid. For your testing purposes you
could just remove the VALID_PAGE() test in mm/memory.c:get_page_map(),
and then gdb should be able to get at the framebuffer.

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