Re: [PATCH RFC bpf-next 2/8] bpf: mark instructions accessing program stack

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 4/13/26 11:28 AM, Alexis Lothoré (eBPF Foundation) wrote:
> In order to prepare to emit KASAN checks in JITed programs, JIT
> compilers need to be aware about whether some load/store instructions
> are targeting the bpf program stack, as those should not be monitored
> (we already have guard pages for that, and it is difficult anyway to
> correctly monitor any kind of data passed on stack).
> 
> To support this need, make the BPF verifier mark the instructions that
> access program stack:
> - add a setter that allows the verifier to mark instructions accessing
>   the program stack
> - add a getter that allows JIT compilers to check whether instructions
>   being JITed are accessing the stack
> 
> Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
> ---
>  include/linux/bpf.h          |  2 ++
>  include/linux/bpf_verifier.h |  2 ++
>  kernel/bpf/core.c            | 10 ++++++++++
>  kernel/bpf/verifier.c        |  7 +++++++
>  4 files changed, 21 insertions(+)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index b4b703c90ca9..774a0395c498 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1543,6 +1543,8 @@ void bpf_jit_uncharge_modmem(u32 size);
>  bool bpf_prog_has_trampoline(const struct bpf_prog *prog);
>  bool bpf_insn_is_indirect_target(const struct bpf_verifier_env *env, const struct bpf_prog *prog,
>  				 int insn_idx);
> +bool bpf_insn_accesses_stack(const struct bpf_verifier_env *env,
> +			     const struct bpf_prog *prog, int insn_idx);
>  #else
>  static inline int bpf_trampoline_link_prog(struct bpf_tramp_link *link,
>  					   struct bpf_trampoline *tr,
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index b148f816f25b..ab99ed4c4227 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -660,6 +660,8 @@ struct bpf_insn_aux_data {
>  	u16 const_reg_map_mask;
>  	u16 const_reg_subprog_mask;
>  	u32 const_reg_vals[10];
> +	/* instruction accesses stack */
> +	bool accesses_stack;
>  };
>  
>  #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 8b018ff48875..340abfdadbed 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1582,6 +1582,16 @@ bool bpf_insn_is_indirect_target(const struct bpf_verifier_env *env, const struc
>  	insn_idx += prog->aux->subprog_start;
>  	return env->insn_aux_data[insn_idx].indirect_target;
>  }
> +
> +bool bpf_insn_accesses_stack(const struct bpf_verifier_env *env,
> +			     const struct bpf_prog *prog, int insn_idx)
> +{
> +	if (!env)
> +		return false;
> +	insn_idx += prog->aux->subprog_start;
> +	return env->insn_aux_data[insn_idx].accesses_stack;
> +}
> +
>  #endif /* CONFIG_BPF_JIT */
>  
>  /* Base function for offset calculation. Needs to go into .text section,
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 1e36b9e91277..7bce4fb4e540 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3502,6 +3502,11 @@ static void mark_indirect_target(struct bpf_verifier_env *env, int idx)
>  	env->insn_aux_data[idx].indirect_target = true;
>  }
>  
> +static void mark_insn_accesses_stack(struct bpf_verifier_env *env, int idx)
> +{
> +	env->insn_aux_data[idx].accesses_stack = true;
> +}
> +
>  #define LR_FRAMENO_BITS	3
>  #define LR_SPI_BITS	6
>  #define LR_ENTRY_BITS	(LR_SPI_BITS + LR_FRAMENO_BITS + 1)
> @@ -6490,6 +6495,8 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>  		else
>  			err = check_stack_write(env, regno, off, size,
>  						value_regno, insn_idx);
> +
> +		mark_insn_accesses_stack(env, insn_idx);

I am not sure this can be done unconditionally here.

It may be possible in different states to have different pointer
types for the affected reg (PTR_TO_STACK in one execution path and say
PTR_TO_MAP_VALUE in another). And if set uncoditionally,
instrumentation may be skipped for legitimate targets.

Maybe reset by default in check_mem_access()?

>  	} else if (reg_is_pkt_pointer(reg)) {
>  		if (t == BPF_WRITE && !may_access_direct_pkt_data(env, NULL, t)) {
>  			verbose(env, "cannot write into packet\n");
>