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

> 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.

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