On 09/15/2014 06:00 PM, Roland Scheidegger wrote:
Am 15.09.2014 08:31, schrieb Kenneth Graunke:
Fredrik's implementation of ARB_vertex_attrib_binding introduced new
gl_vertex_attrib_array and gl_vertex_buffer_binding structures, and
converted Mesa's older gl_client_array to be derived state. Ultimately,
we'd like to drop gl_client_array and use those structures directly.
One hitch is that gl_client_array::_MaxElement doesn't correspond to
either structure (unlike every other field), so we'd have to figure out
where to store it. The _MaxElement computation uses values from both
structures, so it doesn't really belong in either place. We could put
it in the VAO, but we'd have to pass it around everywhere.
It turns out that it's only used when ctx->Const.CheckArrayBounds is
set, which is only set by the (rarely used) classic swrast driver.
It appears that drivers/x11 used to set it as well, which was intended
to avoid segmentation faults on out-of-bounds memory access in the X
server (probably for indirect GLX clients). However, ajax deleted that
code in 2010 (commit 1ccef926be46dce3b6b5c76e812e2fae4e205ce7).
The bounds checking apparently doesn't actually work, either. Non-VBO
attributes arbitrarily set _MaxElement to 2 * 1000 * 1000 * 1000.
vbo_save_draw and vbo_exec_draw remark /* ??? */ when setting it, and
the i965 code contains a comment noting that _MaxElement is often bogus.
Well there's not much you can do for the non-vbo case, since you simply
don't know how large that buffer pointed to by that client pointer you
were given by the app is...
Given that the code is complex, rarely used, and dubiously functional,
it doesn't seem worth maintaining going forward. This patch drops it.
This will probably mean the classic swrast driver may begin crashing on
out of bounds vertex buffer access in some cases, but I believe that is
allowed by OpenGL (and probably happened for non-VBO accesses anyway).
There do not appear to be any Piglit regressions, either.
Signed-off-by: Kenneth Graunke <kenn...@whitecape.org>
---
src/mesa/drivers/dri/i965/brw_draw_upload.c | 5 +--
src/mesa/drivers/dri/swrast/swrast.c | 3 --
src/mesa/main/api_validate.c | 66 -----------------------------
src/mesa/main/arrayobj.c | 46 --------------------
src/mesa/main/arrayobj.h | 4 --
src/mesa/main/attrib.c | 1 -
src/mesa/main/context.c | 3 --
src/mesa/main/mtypes.h | 10 -----
src/mesa/main/state.c | 5 ---
src/mesa/main/varray.c | 9 +---
src/mesa/main/varray.h | 33 ---------------
src/mesa/vbo/vbo_exec_array.c | 26 +++---------
src/mesa/vbo/vbo_exec_draw.c | 2 -
src/mesa/vbo/vbo_save_draw.c | 1 -
src/mesa/vbo/vbo_split_copy.c | 1 -
15 files changed, 9 insertions(+), 206 deletions(-)
Hi Brian, Roland, Eric, all...
What do you think of this idea? Good idea? Terrible idea?
In theory, this seems like a reasonable idea for software drivers, but
it doesn't appear that softpipe/llvmpipe use this code, so I'm not sure
if it's worth maintaining just for swrast...
draw does its own validation (according to more strict d3d10 rules,
even), as long as everything comes in nice buffers with known sizes this
isn't much of a problem (and it doesn't need to be done as a separate pass).
I am pretty sure though any app used to be able to crash the X server
(when using indirect rendering) when not doing bounds checking pretty
easily, but maybe that vanished at some point somehow (luckily, I never
had to look at indirect rendering for years...).
I think at some point it was also useful for debugging (so you could
more easily see where that weird segfault was coming from) though again
of course it did nothing for non-vbo arrays. classic swrast could
probably reimplement this on its own if nothing else uses it anymore. Of
course real hw nowadays you just give the buffer sizes and the hw will
make sure nothing is fetched outside bounds on its own.
So, for me dropping this looks ok, but I'm not really working much in
that area nowadays.
As Eric pointed out, it doesn't look like there's any glDrawElements or
VBO code getting used in the X server (the glDrawElements call gets
"unwound" in the client-side GLX code, AFAICT).
As it is, the CheckArrayBounds code checks for invalid array indexes at
draw-validation time. If a bad index is found, the whole draw is
discarded. Alternately, a driver/device can discard individual
primitives while drawing if an array index is out of bounds (if the
vertex fetch fails, skip the prim). The TnL (src/tnl/) module has never
had any support for per-vertex bounds checking but I believe all the
gallium drivers (should) handle it. I think this behavior is what's
expected of OpenGL/Direct3D nowadays so the CheckArrayBounds approach
isn't very good.
In piglit we have tests/spec/arb_robustness/draw-vbo-bounds.c. It's not
included in all.py though so it's probably seldom run. I tested it with
the software drivers (non-GLX mode):
swrast: failed assertion, implementation error: bad datatype 0x1010101
in interpolate_int_colors, plus valgrind errors in the VBO code. But if
I enable ctx->Const.CheckArrayBounds, I get warnings but no crashes.
llvmpipe: failed assertion, lp_setup_tri.c:495:do_triangle_ccw
softpipe: pass
I just tried this piglit test in Ubuntu 14 with LIBGL_ALWAYS_INDIRECT
and I get a glx protocol error (no X crash). But that's using the
llvmpipe driver.
Anyway, I think this code can be removed. Ideally, some index checking
would be added to the tnl code to try to avoid crashes, but I doubt
anyone wants to bother with it.
Reviewed-by: Brian Paul <bri...@vmware.com>
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