Re: [PATCH bpf v2 2/2] selftests/bpf: Check per-CPU address resolution per register

On 8/14/26 2:20 PM, [email protected] wrote:
>> diff --git a/tools/testing/selftests/bpf/progs/verifier_percpu_addr.c b/tools/testing/selftests/bpf/progs/verifier_percpu_addr.c
>> new file mode 100644
>> index 000000000000..962faea8ef90
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/progs/verifier_percpu_addr.c
>> @@ -0,0 +1,114 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +#include <vmlinux.h>
>> +#include <bpf/bpf_helpers.h>
>> +#include "bpf_misc.h"
>> +
>> +int percpu_data SEC(".percpu");
>> +
>> +#if defined(__TARGET_ARCH_x86)
>> +
>> +/*
>> + * An ld_imm64 of a per-CPU map value is followed by a mov_percpu_addr that
>> + * reuses the same register, so the register the address lands in decides how
>> + * the JIT encodes the add. On x86 R5, R7, R8 and R9 are the extended
>> + * registers, whose high bit needs REX.R because the destination sits in
>> + * ModRM.reg. Check one program per extended register, since getting the
>> + * prefix wrong resolves the address into whichever register shares the low
>> + * three bits instead.
>> + */
>> +
>> +SEC("raw_tp")
>> +__description("per-CPU address into r5")
>> +__success
>> +__arch_x86_64
>> +__jited("	addq	%gs:{{.*}}, %r8")
>> +__naked void percpu_addr_into_r5(void)
>> +{
>> +	asm volatile ("					\
>> +	r5 = %[percpu_data] ll;				\
>> +	r0 = *(u32 *)(r5 + 0);				\
>> +	exit;						\
>> +"	:
>> +	: __imm_addr(percpu_data)
>> +	: __clobber_all);
>> +}
> This isn't a bug, but would it be tidier to move the percpu_data
> declaration inside the __TARGET_ARCH_x86 block?
>
> Currently percpu_data is declared above the arch guard, but its only
> readers (percpu_addr_into_r1/r5/r7/r8/r9) are inside the guard:
>
> +int percpu_data SEC(".percpu");
> +
> +#if defined(__TARGET_ARCH_x86)
> +
> +/* ... test functions using percpu_data ... */
>
> On every other target the object still carries a .percpu section, so
> libbpf still creates an unused percpu data map for it.

That's a good point, will address this in v3

Thx,
-Vineet

>
> Comparable files in the same directory keep objects used only by guarded
> programs inside the guard. verifier_private_stack.c declares its 'struct
> elem' and 'array SEC(".maps")' after the '#if defined(__TARGET_ARCH_x86)
> || defined(__TARGET_ARCH_arm64)' guard, and verifier_ldsx.c declares its
> 'arena SEC(".maps")' map inside its arch guard.
>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/31839526403