Messages in this thread | | | Date | Mon, 26 Jun 2023 23:42:51 +0200 | From | Sebastian Sumpf <> | Subject | Re: [Bug report] __arch_hweight32/64 x86 |
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Hello Borislav,
On 6/26/23 23:06, Borislav Petkov wrote: > Hello, > > On Mon, Jun 26, 2023 at 10:49:44PM +0200, Sebastian Sumpf wrote: >> The 'ALTERNATIVE' macro checks for the popcnt feature. In case this fails >> the '__sw_hweight' C-function is called from inline assembly with rax and >> rdi as intput/output operands. However, the code does not contain a clobber >> list of any callee safe registers that might be touched by the >> '__sw_height64' C-function.
>> Which registers are those? Can you be more specific?
Registers that are free to use by a called C-function are: rcx, rdx, rsi, rdi, and r8-r11 for x86_64. See the "System V Application Binary Interface AMD64 Architecture Processor Supplement" [1] page 21. x86_32 I would have to look up.
>> Therefore, these registers will not be restored >> upon function return by the compiler. This leads to varying >> exceptions/bad behavior caused by the thus corrupted registers later >> on. > > How do I reproduce what you're observing so that I can take a look?
This is hard to tell, I would disable the " X86_FEATURE_POPCNT" feature and use the ' CONFIG_ARCH_HAS_FAST_MULTIPLIER' option in order to use the multiplier implementation in '__sw_hweight64' in 'lib/hweight.c' At least that is what triggered it here.
Regards,
Sebastian
[1] http://www.uclibc.org/docs/psABI-x86_64.pdf
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