linux-kernel - Re: [PATCH bpf-next v11 04/12] bpf: support fsession for bpf_session_is

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Message-Id: <ccb153dc349dfbbced71546e26de91c4e8094a0c515c3e2a9336dfa1a2f0a607@mail.kernel.org>
Date: Fri, 23 Jan 2026 08:07:44 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: menglong8.dong@...il.com,andrii@...nel.org
Cc: daniel@...earbox.net,martin.lau@...ux.dev,eddyz87@...il.com,song@...nel.org,yonghong.song@...ux.dev,john.fastabend@...il.com,kpsingh@...nel.org,sdf@...ichev.me,haoluo@...gle.com,jolsa@...nel.org,davem@...emloft.net,dsahern@...nel.org,tglx@...utronix.de,mingo@...hat.com,jiang.biao@...ux.dev,bp@...en8.de,dave.hansen@...ux.intel.com,x86@...nel.org,hpa@...or.com,bpf@...r.kernel.org,netdev@...r.kernel.org,linux-kernel@...r.kernel.org,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v11 04/12] bpf: support fsession for bpf_session_is_return

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 41228b0add52..29eecd79352e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1229,6 +1229,8 @@ enum {
>  #endif
>  };
>
> +#define BPF_TRAMP_IS_RETURN_SHIFT	63
> +
>  struct bpf_tramp_links {

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 19fd16697262..2ae8f29b854f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -23011,6 +23011,19 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  		   desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
>  		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
>  		*cnt = 1;
> +	} else if (desc->func_id == special_kfunc_list[KF_bpf_session_is_return] &&
> +		   env->prog->expected_attach_type == BPF_TRACE_FSESSION) {
> +		/*
> +		 * inline the bpf_session_is_return() for fsession:
> +		 *   bool bpf_session_is_return(void *ctx)
> +		 *   {
> +		 *       return (((u64 *)ctx)[-1] >> BPF_TRAMP_IS_RETURN_SHIFT) & 1;
> +		 *   }
> +		 */
> +		insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +		insn_buf[1] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, BPF_TRAMP_IS_RETURN_SHIFT);
> +		insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
> +		*cnt = 3;
> +	}

The inline implementation reads bit 63 of ctx[-1] to check the is_return
flag. However, looking at the x86 trampoline code in
arch/x86/net/bpf_jit_comp.c, the trampoline only stores nr_regs in ctx[-1]:

    emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_regs);
    emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -nregs_off);

The is_return flag (bit 63) is never set by the current trampoline code.
This means bpf_session_is_return() will always return 0 for fsession
programs, even when executing at the return point.

I see this is fixed later in the series by commit ba766b26e776 ("bpf,x86:
add fsession support for x86_64") which adds:

    func_meta |= (1ULL << BPF_TRAMP_IS_RETURN_SHIFT);
    if (bpf_fsession_cnt(tlinks))
        emit_store_stack_imm64(&prog, BPF_REG_0, -func_meta_off, func_meta);

Should this commit be squashed with the x86 trampoline changes to ensure
the feature works correctly at each point in the series?

[ ... ]


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