This patch fixes two issues present in the current function for encoding
arm64 logical immediates when using the 32-bit variants of instructions.
First, the code does not correctly reject an all-ones 32-bit immediate,
and returns an undefined instruction encoding.
Second, the code incorrectly rejects some 32-bit immediates that are
actually encodable as logical immediates. The root cause is that the code
uses a default mask of 64-bit all-ones, even for 32-bit immediates.
This causes an issue later on when the default mask is used to fill the
top bits of the immediate with ones, shown here:
/*
* Pattern: 0..01..10..01..1
*
* Fill the unused top bits with ones, and check if
* the result is a valid immediate (all ones with a
* contiguous ranges of zeroes).
*/
imm |= ~mask;
if (!range_of_ones(~imm))
return AARCH64_BREAK_FAULT;
To see the problem, consider an immediate of the form 0..01..10..01..1,
where the upper 32 bits are zero, such as 0x80000001. The code checks
if ~(imm | ~mask) contains a range of ones: the incorrect mask yields
1..10..01..10..0, which fails the check; the correct mask yields
0..01..10..0, which succeeds.
The fix for both issues is to generate a correct mask based on the
instruction immediate size, and use the mask to check for all-ones,
all-zeroes, and values wider than the mask.
Currently, arch/arm64/kvm/va_layout.c is the only user of this function,
which uses 64-bit immediates and therefore won't trigger these bugs.
We tested the new code against llvm-mc with all 1,302 encodable 32-bit
logical immediates and all 5,334 encodable 64-bit logical immediates.
Fixes: ef3935eeeb ("arm64: insn: Add encoder for bitwise operations using literals")
Suggested-by: Will Deacon <will@kernel.org>
Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200508181547.24783-2-luke.r.nels@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>
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