[PATCH bpf RESEND v2 0/2] bpf: Fix memory access flags in helper prototypes

[PATCH bpf RESEND v2 0/2] bpf: Fix memory access flags in helper prototypes

[Zesen Liu]

Hi,

This series adds missing memory access flags (MEM_RDONLY or MEM_WRITE) to
several bpf helper function prototypes that use ARG_PTR_TO_MEM but lack the
correct flag. It also adds a new check in verifier to ensure the flag is
specified.

Missing memory access flags in helper prototypes can lead to critical
correctness issues when the verifier tries to perform code optimization.
After commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type
tracking"), the verifier relies on the memory access flags, rather than
treating all arguments in helper functions as potentially modifying the
pointed-to memory.

Using ARG_PTR_TO_MEM alone without flags does not make sense because:

- If the helper does not change the argument, missing MEM_RDONLY causes the
   verifier to incorrectly reject a read-only buffer.
- If the helper does change the argument, missing MEM_WRITE causes the
   verifier to incorrectly assume the memory is unchanged, leading to
   errors in code optimization.

We have already seen several reports regarding this:

- commit ac44dcc788b9 ("bpf: Fix verifier assumptions of bpf_d_path's
   output buffer") adds MEM_WRITE to bpf_d_path;
- commit 2eb7648558a7 ("bpf: Specify access type of bpf_sysctl_get_name
   args") adds MEM_WRITE to bpf_sysctl_get_name.

This series looks through all prototypes in the kernel and completes the
flags. It also adds a new check (check_func_proto) in
verifier.c to statically restrict ARG_PTR_TO_MEM from appearing without
memory access flags. 

Changelog
=========

v2:
  - Add missing MEM_RDONLY flags to protos with ARG_PTR_TO_FIXED_SIZE_MEM.

Thanks,

Zesen Liu

---
Zesen Liu (2):
      bpf: Fix memory access flags in helper prototypes
      bpf: Require ARG_PTR_TO_MEM with memory flag

 kernel/bpf/helpers.c     |  2 +-
 kernel/bpf/syscall.c     |  2 +-
 kernel/bpf/verifier.c    | 17 +++++++++++++++++
 kernel/trace/bpf_trace.c |  6 +++---
 net/core/filter.c        | 20 ++++++++++----------
 5 files changed, 32 insertions(+), 15 deletions(-)
---
base-commit: ab86d0bf01f6d0e37fd67761bb62918321b64efc
change-id: 20251220-helper_proto-fb6e64182467

Best regards,
-- 
Zesen Liu <ftyghome@gmail.com>

[PATCH bpf RESEND v2 1/2] bpf: Fix memory access flags in helper prototypes

[Zesen Liu]

After commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking"),
the verifier started relying on the access type flags in helper
function prototypes to perform memory access optimizations.

Currently, several helper functions utilizing ARG_PTR_TO_MEM lack the
corresponding MEM_RDONLY or MEM_WRITE flags. This omission causes the
verifier to incorrectly assume that the buffer contents are unchanged
across the helper call. Consequently, the verifier may optimize away
subsequent reads based on this wrong assumption, leading to correctness
issues.

For bpf_get_stack_proto_raw_tp, the original MEM_RDONLY was incorrect
since the helper writes to the buffer. Change it to ARG_PTR_TO_UNINIT_MEM
which correctly indicates write access to potentially uninitialized memory.

Similar issues were recently addressed for specific helpers in commit
ac44dcc788b9 ("bpf: Fix verifier assumptions of bpf_d_path's output buffer")
and commit 2eb7648558a7 ("bpf: Specify access type of bpf_sysctl_get_name args").

Fix these prototypes by adding the correct memory access flags.

Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking")
Co-developed-by: Shuran Liu <electronlsr@gmail.com>
Signed-off-by: Shuran Liu <electronlsr@gmail.com>
Co-developed-by: Peili Gao <gplhust955@gmail.com>
Signed-off-by: Peili Gao <gplhust955@gmail.com>
Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Zesen Liu <ftyghome@gmail.com>
---
 kernel/bpf/helpers.c     |  2 +-
 kernel/bpf/syscall.c     |  2 +-
 kernel/trace/bpf_trace.c |  6 +++---
 net/core/filter.c        | 20 ++++++++++----------
 4 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index db72b96f9c8c..f66284f8ec2c 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1077,7 +1077,7 @@ const struct bpf_func_proto bpf_snprintf_proto = {
 	.func		= bpf_snprintf,
 	.gpl_only	= true,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_MEM_OR_NULL,
+	.arg1_type	= ARG_PTR_TO_MEM_OR_NULL | MEM_WRITE,
 	.arg2_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg3_type	= ARG_PTR_TO_CONST_STR,
 	.arg4_type	= ARG_PTR_TO_MEM | PTR_MAYBE_NULL | MEM_RDONLY,
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4ff82144f885..ee116a3b7baf 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -6407,7 +6407,7 @@ static const struct bpf_func_proto bpf_kallsyms_lookup_name_proto = {
 	.func		= bpf_kallsyms_lookup_name,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_MEM,
+	.arg1_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
 	.arg2_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg3_type	= ARG_ANYTHING,
 	.arg4_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_UNINIT | MEM_WRITE | MEM_ALIGNED,
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index fe28d86f7c35..59c2394981c7 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1022,7 +1022,7 @@ const struct bpf_func_proto bpf_snprintf_btf_proto = {
 	.func		= bpf_snprintf_btf,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_MEM,
+	.arg1_type	= ARG_PTR_TO_MEM | MEM_WRITE,
 	.arg2_type	= ARG_CONST_SIZE,
 	.arg3_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
 	.arg4_type	= ARG_CONST_SIZE,
@@ -1526,7 +1526,7 @@ static const struct bpf_func_proto bpf_read_branch_records_proto = {
 	.gpl_only       = true,
 	.ret_type       = RET_INTEGER,
 	.arg1_type      = ARG_PTR_TO_CTX,
-	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
+	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL | MEM_WRITE,
 	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type      = ARG_ANYTHING,
 };
@@ -1661,7 +1661,7 @@ static const struct bpf_func_proto bpf_get_stack_proto_raw_tp = {
 	.gpl_only	= true,
 	.ret_type	= RET_INTEGER,
 	.arg1_type	= ARG_PTR_TO_CTX,
-	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
+	.arg2_type	= ARG_PTR_TO_UNINIT_MEM,
 	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 };
diff --git a/net/core/filter.c b/net/core/filter.c
index 616e0520a0bb..18174e0d3fcf 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6399,7 +6399,7 @@ static const struct bpf_func_proto bpf_xdp_fib_lookup_proto = {
 	.gpl_only	= true,
 	.ret_type	= RET_INTEGER,
 	.arg1_type      = ARG_PTR_TO_CTX,
-	.arg2_type      = ARG_PTR_TO_MEM,
+	.arg2_type      = ARG_PTR_TO_MEM | MEM_WRITE,
 	.arg3_type      = ARG_CONST_SIZE,
 	.arg4_type	= ARG_ANYTHING,
 };
@@ -6454,7 +6454,7 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
 	.gpl_only	= true,
 	.ret_type	= RET_INTEGER,
 	.arg1_type      = ARG_PTR_TO_CTX,
-	.arg2_type      = ARG_PTR_TO_MEM,
+	.arg2_type      = ARG_PTR_TO_MEM | MEM_WRITE,
 	.arg3_type      = ARG_CONST_SIZE,
 	.arg4_type	= ARG_ANYTHING,
 };
@@ -8008,9 +8008,9 @@ static const struct bpf_func_proto bpf_tcp_raw_gen_syncookie_ipv4_proto = {
 	.gpl_only	= true, /* __cookie_v4_init_sequence() is GPL */
 	.pkt_access	= true,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM,
+	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_RDONLY,
 	.arg1_size	= sizeof(struct iphdr),
-	.arg2_type	= ARG_PTR_TO_MEM,
+	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
 	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 };
 
@@ -8040,9 +8040,9 @@ static const struct bpf_func_proto bpf_tcp_raw_gen_syncookie_ipv6_proto = {
 	.gpl_only	= true, /* __cookie_v6_init_sequence() is GPL */
 	.pkt_access	= true,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM,
+	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_RDONLY,
 	.arg1_size	= sizeof(struct ipv6hdr),
-	.arg2_type	= ARG_PTR_TO_MEM,
+	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
 	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 };
 
@@ -8060,9 +8060,9 @@ static const struct bpf_func_proto bpf_tcp_raw_check_syncookie_ipv4_proto = {
 	.gpl_only	= true, /* __cookie_v4_check is GPL */
 	.pkt_access	= true,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM,
+	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_RDONLY,
 	.arg1_size	= sizeof(struct iphdr),
-	.arg2_type	= ARG_PTR_TO_FIXED_SIZE_MEM,
+	.arg2_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_RDONLY,
 	.arg2_size	= sizeof(struct tcphdr),
 };
 
@@ -8084,9 +8084,9 @@ static const struct bpf_func_proto bpf_tcp_raw_check_syncookie_ipv6_proto = {
 	.gpl_only	= true, /* __cookie_v6_check is GPL */
 	.pkt_access	= true,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM,
+	.arg1_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_RDONLY,
 	.arg1_size	= sizeof(struct ipv6hdr),
-	.arg2_type	= ARG_PTR_TO_FIXED_SIZE_MEM,
+	.arg2_type	= ARG_PTR_TO_FIXED_SIZE_MEM | MEM_RDONLY,
 	.arg2_size	= sizeof(struct tcphdr),
 };
 #endif /* CONFIG_SYN_COOKIES */

-- 
2.43.0

[PATCH bpf RESEND v2 2/2] bpf: Require ARG_PTR_TO_MEM with memory flag

[Zesen Liu]

Add check to ensure that ARG_PTR_TO_MEM is used with either MEM_WRITE or
MEM_RDONLY.

Using ARG_PTR_TO_MEM alone without tags does not make sense because:

- If the helper does not change the argument, missing MEM_RDONLY causes the
verifier to incorrectly reject a read-only buffer.
- If the helper does change the argument, missing MEM_WRITE causes the
verifier to incorrectly assume the memory is unchanged, leading to errors
in code optimization.

Co-developed-by: Shuran Liu <electronlsr@gmail.com>
Signed-off-by: Shuran Liu <electronlsr@gmail.com>
Co-developed-by: Peili Gao <gplhust955@gmail.com>
Signed-off-by: Peili Gao <gplhust955@gmail.com>
Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Zesen Liu <ftyghome@gmail.com>
---
 kernel/bpf/verifier.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f0ca69f888fa..c7ebddb66385 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10349,10 +10349,27 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
 	return true;
 }
 
+static bool check_mem_arg_rw_flag_ok(const struct bpf_func_proto *fn)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++) {
+		enum bpf_arg_type arg_type = fn->arg_type[i];
+
+		if (base_type(arg_type) != ARG_PTR_TO_MEM)
+			continue;
+		if (!(arg_type & (MEM_WRITE | MEM_RDONLY)))
+			return false;
+	}
+
+	return true;
+}
+
 static int check_func_proto(const struct bpf_func_proto *fn, int func_id)
 {
 	return check_raw_mode_ok(fn) &&
 	       check_arg_pair_ok(fn) &&
+		   check_mem_arg_rw_flag_ok(fn) &&
 	       check_btf_id_ok(fn) ? 0 : -EINVAL;
 }
 

-- 
2.43.0

Re: [PATCH bpf RESEND v2 1/2] bpf: Fix memory access flags in helper prototypes

[Eduard Zingerman]

On Sun, 2026-01-18 at 16:16 +0800, Zesen Liu wrote:
> After commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking"),
> the verifier started relying on the access type flags in helper
> function prototypes to perform memory access optimizations.
>
> Currently, several helper functions utilizing ARG_PTR_TO_MEM lack the
> corresponding MEM_RDONLY or MEM_WRITE flags. This omission causes the
> verifier to incorrectly assume that the buffer contents are unchanged
> across the helper call. Consequently, the verifier may optimize away
> subsequent reads based on this wrong assumption, leading to correctness
> issues.
>
> For bpf_get_stack_proto_raw_tp, the original MEM_RDONLY was incorrect
> since the helper writes to the buffer. Change it to ARG_PTR_TO_UNINIT_MEM
> which correctly indicates write access to potentially uninitialized memory.
>
> Similar issues were recently addressed for specific helpers in commit
> ac44dcc788b9 ("bpf: Fix verifier assumptions of bpf_d_path's output buffer")
> and commit 2eb7648558a7 ("bpf: Specify access type of bpf_sysctl_get_name args").
>
> Fix these prototypes by adding the correct memory access flags.
>
> Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking")
> Co-developed-by: Shuran Liu <electronlsr@gmail.com>
> Signed-off-by: Shuran Liu <electronlsr@gmail.com>
> Co-developed-by: Peili Gao <gplhust955@gmail.com>
> Signed-off-by: Peili Gao <gplhust955@gmail.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Zesen Liu <ftyghome@gmail.com>
> ---

I looked trough the helpers annotated with MEM_WRITE in this patch,
indeed the write annotation is missing from these helpers.

In conjunction with the following logic in verifier.c:check_func_arg:

        case ARG_PTR_TO_MEM:
                /* The access to this pointer is only checked when we hit the
                 * next is_mem_size argument below.
                 */
                meta->raw_mode = arg_type & MEM_UNINIT;
                if (arg_type & MEM_FIXED_SIZE) {
                        err = check_helper_mem_access(env, regno, fn->arg_size[arg],
                                                      arg_type & MEM_WRITE ? BPF_WRITE : BPF_READ,
						      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
						      // arguments considered read-only by default
                                                      false, meta);
                        if (err)
                                return err;
                        if (arg_type & MEM_ALIGNED)
                                err = check_ptr_alignment(env, reg, 0, fn->arg_size[arg], true);
                }
                break;

This patch fixes a real problem.

[...]

> index fe28d86f7c35..59c2394981c7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c

[...]

> @@ -1526,7 +1526,7 @@ static const struct bpf_func_proto bpf_read_branch_records_proto = {
>  	.gpl_only       = true,
>  	.ret_type       = RET_INTEGER,
>  	.arg1_type      = ARG_PTR_TO_CTX,
> -	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
> +	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL | MEM_WRITE,
>  	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>  	.arg4_type      = ARG_ANYTHING,
>  };
> @@ -1661,7 +1661,7 @@ static const struct bpf_func_proto bpf_get_stack_proto_raw_tp = {
>  	.gpl_only	= true,
>  	.ret_type	= RET_INTEGER,
>  	.arg1_type	= ARG_PTR_TO_CTX,
> -	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
> +	.arg2_type	= ARG_PTR_TO_UNINIT_MEM,

Q: why ARG_PTR_TO_UNINIT_MEM here, but not for a previous function and
   not for snprintf variants?

>  	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
>  	.arg4_type	= ARG_ANYTHING,
>  };

[...]

Re: [PATCH bpf RESEND v2 2/2] bpf: Require ARG_PTR_TO_MEM with memory flag

[Eduard Zingerman]

On Sun, 2026-01-18 at 16:16 +0800, Zesen Liu wrote:
> Add check to ensure that ARG_PTR_TO_MEM is used with either MEM_WRITE or
> MEM_RDONLY.
> 
> Using ARG_PTR_TO_MEM alone without tags does not make sense because:
> 
> - If the helper does not change the argument, missing MEM_RDONLY causes the
> verifier to incorrectly reject a read-only buffer.
> - If the helper does change the argument, missing MEM_WRITE causes the
> verifier to incorrectly assume the memory is unchanged, leading to errors
> in code optimization.
> 
> Co-developed-by: Shuran Liu <electronlsr@gmail.com>
> Signed-off-by: Shuran Liu <electronlsr@gmail.com>
> Co-developed-by: Peili Gao <gplhust955@gmail.com>
> Signed-off-by: Peili Gao <gplhust955@gmail.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Zesen Liu <ftyghome@gmail.com>
> ---

Note: this patch no longer applies properly because of the change in
      the check_func_proto() signature.

[...]

Re: [PATCH bpf RESEND v2 1/2] bpf: Fix memory access flags in helper prototypes

[Zesen Liu]

[...]

> On Jan 20, 2026, at 04:24, Eduard Zingerman <eddyz87@gmail.com> wrote:
> 
> Q: why ARG_PTR_TO_UNINIT_MEM here, but not for a previous function and
>   not for snprintf variants?

For bpf_get_stack_proto_raw_tp, I chose ARG_PTR_TO_UNINIT_MEM to be consistent with its siblings:

• bpf_get_stack_proto_tp (bpf_trace.c:1425)
• bpf_get_stack_proto (stackmap.c:525)
• bpf_get_stack_sleepable_proto (stackmap.c:541)

All of these are wrappers around the same core function bpf_get_stack() / __bpf_get_stack(), passing the buffer with identical semantics, and all use ARG_PTR_TO_UNINIT_MEM.

Re: [PATCH bpf RESEND v2 2/2] bpf: Require ARG_PTR_TO_MEM with memory flag

[Zesen Liu]

[...]

> On Jan 20, 2026, at 04:25, Eduard Zingerman <eddyz87@gmail.com> wrote:
> 
> Note: this patch no longer applies properly because of the change in
>      the check_func_proto() signature.

Thanks for pointing this out. I will rebase to bpf-next in v3 to
address the check_func_proto() signature changes.

Re: [PATCH bpf RESEND v2 1/2] bpf: Fix memory access flags in helper prototypes

[Eduard Zingerman]

On Tue, 2026-01-20 at 11:39 +0800, Zesen Liu wrote:
> [...]
> 
> > On Jan 20, 2026, at 04:24, Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > Q: why ARG_PTR_TO_UNINIT_MEM here, but not for a previous function and
> >   not for snprintf variants?
> 
> For bpf_get_stack_proto_raw_tp, I chose ARG_PTR_TO_UNINIT_MEM to be consistent with its siblings:
> 
> • bpf_get_stack_proto_tp (bpf_trace.c:1425)
> • bpf_get_stack_proto (stackmap.c:525)
> • bpf_get_stack_sleepable_proto (stackmap.c:541)
> 
> All of these are wrappers around the same core function bpf_get_stack() / __bpf_get_stack(), passing the buffer with identical semantics, and all use ARG_PTR_TO_UNINIT_MEM.

Ack, makes sense, thank you for explaining.