Re: [PATCH bpf-next v7 04/11] bpf: support fsession for bpf_session_is_return

[Menglong Dong]

On 2026/1/7 15:11 bot+bpf-ci@xxxxxxxxxx write:
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 41228b0add52..95248b0d28ab 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1229,6 +1229,9 @@ enum {
> >  #endif
> >  };
> >
> > +#define BPF_TRAMP_M_NR_ARGS	0
> > +#define BPF_TRAMP_M_IS_RETURN	8
> 
> [ ... ]
> 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index bfff3f84fd91..d3709edd0e51 100644
> 
> [ ... ]
> 
> > @@ -22558,6 +22564,13 @@ 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) {
> > +		/* Load nr_args from ctx - 8 */
> > +		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_M_IS_RETURN);
> > +		insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
> > +		*cnt = 3;
>                 ^^^^
> 
> This inlined code reads the BPF_TRAMP_M_IS_RETURN bit (bit 8) from ctx[-1]
> to determine if the program is running at function exit. However, looking
> at arch/x86/net/bpf_jit_comp.c, the trampoline only stores nr_regs to
> ctx[-8] at this point in the series:
> 
>     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 BPF_TRAMP_M_IS_RETURN flag is not being set by the trampoline code.
> Is this intentional? The arch code that sets this bit appears to be added
> in a later commit in the series (93d4641b9513 "bpf,x86: add fsession
> support for x86_64").
> 
> At this commit, bpf_session_is_return() for fsession programs will always
> return false, even during fexit execution, because bit 8 is never set.

Yeah, this is intentional. The fsession is architecture specific feature.
For the architecture that doesn't support this feature, the "is_return"
flag will never be set.

After the commit (93d4641b9513 "bpf,x86: add fsession support for x86_64"),
x86_64 support this feature, and the flag will be set for fsession.

> 
> > +	}
> >
> >  	if (env->insn_aux_data[insn_idx].arg_prog) {
> 
> [ ... ]
> 
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 736b32cf2195..9d3bf3bbe8f6 100644
> 
> [ ... ]
> 
> > @@ -3314,6 +3314,12 @@ static u64 bpf_uprobe_multi_entry_ip(struct bpf_run_ctx *ctx)
> >  }
> >  #endif /* CONFIG_UPROBES */
> >
> > +bool bpf_fsession_is_return(void *ctx)
> > +{
> > +	/* This helper call is inlined by verifier. */
> > +	return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
> > +}
> 
> The same issue exists here in bpf_fsession_is_return() - it reads bit 8
> which is not set by the trampoline at this point in the series. The
> comment indicates this is inlined by the verifier, but both code paths
> have the same problem.

The logic here is right, as I said above.

> 
> [ ... ]
> 
> 
> ---
> 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/20773233136
>