| From | Andrew Cooper <> | Subject | Re: [PATCH 05/29] x86: Base IBT bits | Date | Fri, 18 Feb 2022 20:49:45 +0000 |
| |
On 18/02/2022 16:49, Peter Zijlstra wrote: > +/* > + * A bit convoluted, but matches both endbr32 and endbr64 without > + * having either as literal in the text. > + */ > +static inline bool is_endbr(const void *addr) > +{ > + unsigned int val = ~*(unsigned int *)addr; > + val |= 0x01000000U; > + return val == ~0xfa1e0ff3; > +}
At this point, I feel I've earned an "I told you so". :)
Clang 13 sees straight through the trickery and generates:
is_endbr: # @is_endbr movl $-16777217, %eax # imm = 0xFEFFFFFF andl (%rdi), %eax cmpl $-98693133, %eax # imm = 0xFA1E0FF3 sete %al retq
Here's one I prepared earlier:
/* * In some cases we need to inspect/insert endbr64 instructions. * * The naive way, mem{cmp,cpy}(ptr, "\xf3\x0f\x1e\xfa", 4), optimises unsafely * by placing 0xfa1e0ff3 in an imm32 operand, and marks a legal indirect * branch target as far as the CPU is concerned. * * gen_endbr64() is written deliberately to avoid the problematic operand, and * marked __const__ as it is safe for the optimiser to hoist/merge/etc. */ static inline uint32_t __attribute_const__ gen_endbr64(void) { uint32_t res;
asm ( "mov $~0xfa1e0ff3, %[res]\n\t" "not %[res]\n\t" : [res] "=&r" (res) );
return res; }
which should be robust against even the most enterprising optimiser.
~Andrew
P.S. Clang IAS had better never get "clever" enough to optimise what it finds in asm statements...
|