[Bug report] __arch_hweight32/64 x86

[Sebastian Sumpf]

Hello kernel developers,

I recently stumbled over a bug in the inline assembly of '__arch_hweight32' and 
'__arch_hweight64'  in 'arch/x86/include/asm/arch_hweight.h' (present in 6.4 as 
well):

! asm (ALTERNATIVE("call __sw_hweight64", "popcntq %1, %0",
! X86_FEATURE_POPCNT)
! 			 : "="REG_OUT (res)
! 			 : REG_IN (w));

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. 
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.

Kind regards,

Sebastian


--
Sebastian Sumpf
Genode Labs

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