RE: [PATCH v5 07/16] x86/module: Deal with GOT based stack cookie load on Clang < 17

[David Laight <David.Laight@ACULAB.COM>]

From: Brian Gerst
> Sent: 05 November 2024 15:58
> 
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Clang versions before 17 will not honour -fdirect-access-external-data
> for the load of the stack cookie emitted into each function's prologue
> and epilogue.
> 
> This is not an issue for the core kernel, as the linker will relax these
> loads into LEA instructions that take the address of __stack_chk_guard
> directly. For modules, however, we need to work around this, by dealing
> with R_X86_64_REX_GOTPCRELX relocations that refer to __stack_chk_guard.
> 
> In this case, given that this is a GOT load, the reference should not
> refer to __stack_chk_guard directly, but to a memory location that holds
> its address. So take the address of __stack_chk_guard into a static
> variable, and fix up the relocations to refer to that.
> 
...
> +#if defined(CONFIG_STACKPROTECTOR) && \
> +    defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION < 170000
> +		case R_X86_64_REX_GOTPCRELX: {
> +			static unsigned long __percpu *const addr = &__stack_chk_guard;
> +
> +			if (sym->st_value != (u64)addr) {
> +				pr_err("%s: Unsupported GOTPCREL relocation\n", me->name);
> +				return -ENOEXEC;
> +			}
> +
> +			val = (u64)&addr + rel[i].r_addend;
> +			fallthrough;
> +		}
> +#endif

Doesn't this depend on the compiler used to compile the module not that
used to compile this code?
(In principle external modules should be able to use a different compiler.)

So the CLANG tests should be replaced by a comment.

	David

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