On 1/4/24 8:37 AM, Eduard Zingerman wrote:
On Wed, 2024-01-03 at 15:26 -0800, Yonghong Song wrote:
With patch set [1], precision backtracing supports register spill/fill
to/from the stack. The patch [2] allows initial imprecise register spill
with content 0. This is a common case for cpuv3 and lower for
initializing the stack variables with pattern
r1 = 0
*(u64 *)(r10 - 8) = r1
and the [2] has demonstrated good verification improvement.
For cpuv4, the initialization could be
*(u64 *)(r10 - 8) = 0
The current verifier marks the r10-8 contents with STACK_ZERO.
Similar to [2], let us permit the above insn to behave like
imprecise register spill which can reduce number of verified states.
The change is in function check_stack_write_fixed_off().
Hi Yonghong,
I agree with this change, but I don't understand under which conditions
current STACK_ZERO logic is sub-optimal.
I tried executing test case from patch #2 w/o applying patch #1 and it passes.
Could you please elaborate / conjure a test case that would fail w/o patch #1?
The logic is similar to
https://lore.kernel.org/all/20231205184248.1502704-9-andrii@xxxxxxxxxx/
STACK_ZERO logic is sub-optimal in some cases only w.r.t. the number of
verifier states. So there is no correctness issue.
Patch 2 is added in response to Andrii's request in
https://lore.kernel.org/all/CAEf4BzaWets3fHUGtctwCNWecR9ASRCO2kFagNy8jJZmPBWYDA@xxxxxxxxxxxxxx/
Since with patch 1 the original STACK_ZERO case is converted to STACK_SPILL,
Patch 2 is added to cover STACK_ZERO case. So with or with patch 1, patch 2
will succeed since it uses STACK_ZERO logic.
Thanks,
Eduard
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