On Tue, Jan 15, 2013 at 04:17:54PM +0000, Chris Wilson wrote:
> In the slow path, we are forced to copy the relocations prior to
> acquiring the struct mutex in order to handle pagefaults. We forgo
> copying the new offsets back into the relocation entries in order to
> prevent a recursive locking bug should we trigger a pagefault whilst
> holding the mutex for the reservations of the execbuffer. Therefore, we
> need to reset the presumed_offsets just in case the objects are rebound
> back into their old locations after relocating for this exexbuffer - if
> that were to happen we would assume the relocations were valid and leave
> the actual pointers to the kernels dangling, instant hang.
>
> Fixes regression from commit bcf50e2775bbc3101932d8e4ab8c7902aa4163b4
> Author: Chris Wilson <ch...@chris-wilson.co.uk>
> Date: Sun Nov 21 22:07:12 2010 +0000
>
> drm/i915: Handle pagefaults in execbuffer user relocations
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55984
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vet...@fwll.ch>
> Cc: stable@vger.kernel.org
Awesome piece of debugging!
> ---
> drivers/gpu/drm/i915/i915_gem_execbuffer.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 4532757..40c062d 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -767,6 +767,8 @@ i915_gem_execbuffer_relocate_slow(struct drm_device *dev,
> total = 0;
> for (i = 0; i < count; i++) {
> struct drm_i915_gem_relocation_entry __user *user_relocs;
> + u64 invalid_offset = (u64)-1;
I'm a bit uneasy with the semantics here, fearing that a random piece of
userspace ORs in a few flags instead of adding them. Could we align this
to 4096 bytes? Or maybe enshrine 0 as our official invalid reloc ...
-Daniel
> + int j;
>
> user_relocs = (void __user *)(uintptr_t)exec[i].relocs_ptr;
>
> @@ -777,6 +779,25 @@ i915_gem_execbuffer_relocate_slow(struct drm_device *dev,
> goto err;
> }
>
> + /* As we do not update the known relocation offsets after
> + * relocating (due to the complexities in lock handling),
> + * we need to mark them as invalid now so that we force the
> + * relocation processing next time. Just in case the target
> + * object is evicted and then rebound into its old
> + * presumed_offset before the next execbuffer - if that
> + * happened we would make the mistake of assuming that the
> + * relocations were valid.
> + */
> + for (j = 0; j < exec[i].relocation_count; j++) {
> + if (copy_to_user(&user_relocs[j].presumed_offset,
> + &invalid_offset,
> + sizeof(invalid_offset))) {
> + ret = -EFAULT;
> + mutex_lock(&dev->struct_mutex);
> + goto err;
> + }
> + }
> +
> reloc_offset[i] = total;
> total += exec[i].relocation_count;
> }
> --
> 1.7.10.4
>
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--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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