Re: [PATCH 05/29] x86: Base IBT bits
From: Andrew Cooper
Date: Fri Feb 18 2022 - 15:49:59 EST
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...