Re: [PATCH bpf-next v4 1/3] bpf, x86: Support fetching AND/OR/XOR atomics in arena

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Wed, Oct 14, 2026 at 05:50 AM Puranjay Mohan <puranjay@kernel.org> wrote:
> @@ -2926,6 +2989,51 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
>  			fallthrough;
>  		case BPF_STX | BPF_PROBE_ATOMIC | BPF_W:
>  		case BPF_STX | BPF_PROBE_ATOMIC | BPF_DW:
> +			if (is_atomic_fetch_op(insn)) {
[...]
> +				/*
> +				 * An access through a number has its address in
> +				 * BPF_REG_AX (see arena_scalar_access()), where the
> +				 * loop below has to save R0.
> +				 */
> +				if (dst_reg == BPF_REG_AX)
> +					return -EOPNOTSUPP;

Reiterating TJ's point differently

can it save R0 with push rax / pop rax in this case, like div/mod do ?
When src_reg is R0 the operand is on the stack: 'or r11, [rsp]'.

Every arena access in Rust is through a number, so after this patch
fetch_or() works in C and doesn't work in Rust.

It's not "as they did before" either.
The verifier used to say
"BPF_ATOMIC stores into R6 scalar is not allowed".
Now the verifier accepts the prog and the load fails with ENOTSUPP
and nothing in the log.

Pls fix and respin patche 1 and 2.

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