[PATCH bpf v2 0/2] Compare stack frames in exact register states

[PATCH bpf v2 0/2] Compare stack frames in exact register states

[Kumar Kartikeya Dwivedi]

regs_exact() compares register values and their ID relationships, but it
does not compare frameno. regsafe() checks frameno for ordinary
PTR_TO_STACK comparisons, while its EXACT path returns through regs_exact()
before reaching that check. Infinite-loop detection can therefore mistake
pointers to the same offset in different stack frames for the same pointer
and reject a finite loop.

Move frameno into bpf_reg_state's type-specific metadata union so the
existing regs_exact() prefix comparison covers it. This avoids a separate
PTR_TO_STACK case and keeps the structure at 80 bytes. Adjust the
states_maybe_looping() comparison boundary for the new layout.

The selftests cover both a live register and a spilled stack pointer. On
the unfixed tree, both programs are rejected with "infinite loop detected".
With the fix, both load and return the expected value.

Changelog:
----------
v1 -> v2
v1: https://lore.kernel.org/bpf/20260914161340.3419141-1-memxor@gmail.com

 * Rebase on bpf/master.
 * Move frameno into the type-specific metadata union so regs_exact()'s
   existing prefix comparison covers it without growing bpf_reg_state.

Kumar Kartikeya Dwivedi (2):
  bpf: Compare stack frames in regs_exact()
  selftests/bpf: Cover frame changes in bounded loops

 include/linux/bpf_verifier.h                  | 16 ++---
 kernel/bpf/states.c                           |  7 +-
 .../selftests/bpf/progs/verifier_loops1.c     | 70 +++++++++++++++++++
 3 files changed, 80 insertions(+), 13 deletions(-)


base-commit: 3b8e5d9f2ed7abf18ebe63b7a6a4a64e1a9c2203
-- 
2.53.0

[PATCH bpf v2 1/2] bpf: Compare stack frames in regs_exact()

[Kumar Kartikeya Dwivedi]

regs_exact() compares the register state up to id, followed by the ID
mappings, but does not compare frameno. The PTR_TO_STACK case in regsafe()
checks frameno separately, which is bypassed when exact comparison is
requested. Consequently, infinite-loop detection can treat pointers to
different stack frames as the same pointer and reject a finite loop.

For example, initialize fp-8 to zero in the caller and to one in the
callee, then pass the caller's fp-8 to the callee as r1:

    loop:
        r0 = *(u64 *)(r1 + 0);
        if r0 != 0 goto done;
        r1 = r10;
        r1 += -8;
        goto loop;
    done:
        exit;

The loop terminates after reading the callee's slot on its second
iteration. At the loop header, however, the only relevant difference is
r1's frameno, so exact comparison incorrectly reports an infinite loop.
The same problem occurs when the pointer is spilled to the stack.

Move frameno into the type-specific metadata union, ahead of id, so the
existing prefix comparison in regs_exact() covers it. Ordinary stack
pointers do not use another union member. Iterator and IRQ stack-slot
states use their dedicated union views and do not need a frame lookup.
This also keeps bpf_reg_state at 80 bytes.

Move the states_maybe_looping() boundary from frameno to precise after the
field relocation. Its prefix comparison continues to cover the complete
value state and now includes frameno.

Continue to ignore precise. Precision marks control whether pruning may
ignore scalar ranges; they do not change the represented values, and exact
comparison already compares those ranges unconditionally. Marks can also
change through backtracking while an ancestor state is still being
explored.

Fixes: d5b892fd607a ("bpf: make infinite loop detection in is_state_visited() exact")
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf_verifier.h | 16 ++++++++--------
 kernel/bpf/states.c          |  7 ++-----
 2 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index a7202b44ab10..17be5f7df35a 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -45,6 +45,14 @@ struct bpf_reg_state {
 	union {
 		/* valid when type == PTR_TO_PACKET */
 		int range;
+		/*
+		 * Inside the callee two registers can be both PTR_TO_STACK like
+		 * R1=fp-8 and R2=fp-8, but one of them points to this function stack
+		 * while another to the caller's stack. To differentiate them 'frameno'
+		 * is used which is an index in bpf_verifier_state->frame[] array
+		 * pointing to bpf_func_state.
+		 */
+		u8 frameno;
 
 		/*
 		 * For CONST_PTR_TO_MAP, PTR_TO_MAP_KEY, PTR_TO_MAP_VALUE and
@@ -155,14 +163,6 @@ struct bpf_reg_state {
 	 * during state comparisons.
 	 */
 	u32 map_uid;
-	/*
-	 * Inside the callee two registers can be both PTR_TO_STACK like
-	 * R1=fp-8 and R2=fp-8, but one of them points to this function stack
-	 * while another to the caller's stack. To differentiate them 'frameno'
-	 * is used which is an index in bpf_verifier_state->frame[] array
-	 * pointing to bpf_func_state.
-	 */
-	u8 frameno;
 	/* if (!precise && SCALAR_VALUE) min/max/tnum don't affect safety */
 	bool precise;
 };
diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
index e4ec007f7fa6..012b82513a3b 100644
--- a/kernel/bpf/states.c
+++ b/kernel/bpf/states.c
@@ -644,10 +644,7 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
 		return range_within(rold, rcur) &&
 		       tnum_in(rold->var_off, rcur->var_off);
 	case PTR_TO_STACK:
-		/* two stack pointers are equal only if they're pointing to
-		 * the same stack frame, since fp-8 in foo != fp-8 in bar
-		 */
-		return regs_exact(rold, rcur, idmap) && rold->frameno == rcur->frameno;
+		return regs_exact(rold, rcur, idmap);
 	case PTR_TO_ARENA:
 		return true;
 	case PTR_TO_INSN:
@@ -1126,7 +1123,7 @@ static bool states_maybe_looping(struct bpf_verifier_state *old,
 	fcur = cur->frame[fr];
 	for (i = 0; i < MAX_BPF_REG; i++)
 		if (memcmp(&fold->regs[i], &fcur->regs[i],
-			   offsetof(struct bpf_reg_state, frameno)))
+			   offsetof(struct bpf_reg_state, precise)))
 			return false;
 	return true;
 }
-- 
2.53.0

Re: [PATCH bpf v2 1/2] bpf: Compare stack frames in regs_exact()

[Eduard Zingerman]

On Fri, 2026-09-18 at 03:13 +0200, Kumar Kartikeya Dwivedi wrote:
> regs_exact() compares the register state up to id, followed by the ID
> mappings, but does not compare frameno. The PTR_TO_STACK case in regsafe()
> checks frameno separately, which is bypassed when exact comparison is
> requested. Consequently, infinite-loop detection can treat pointers to
> different stack frames as the same pointer and reject a finite loop.
> 
> For example, initialize fp-8 to zero in the caller and to one in the
> callee, then pass the caller's fp-8 to the callee as r1:
> 
>     loop:
>         r0 = *(u64 *)(r1 + 0);
>         if r0 != 0 goto done;
>         r1 = r10;
>         r1 += -8;
>         goto loop;
>     done:
>         exit;
> 
> The loop terminates after reading the callee's slot on its second
> iteration. At the loop header, however, the only relevant difference is
> r1's frameno, so exact comparison incorrectly reports an infinite loop.
> The same problem occurs when the pointer is spilled to the stack.
> 
> Move frameno into the type-specific metadata union, ahead of id, so the
> existing prefix comparison in regs_exact() covers it. Ordinary stack
> pointers do not use another union member. Iterator and IRQ stack-slot
> states use their dedicated union views and do not need a frame lookup.
> This also keeps bpf_reg_state at 80 bytes.
> 
> Move the states_maybe_looping() boundary from frameno to precise after the
> field relocation. Its prefix comparison continues to cover the complete
> value state and now includes frameno.
> 
> Continue to ignore precise. Precision marks control whether pruning may
> ignore scalar ranges; they do not change the represented values, and exact
> comparison already compares those ranges unconditionally. Marks can also
> change through backtracking while an ancestor state is still being
> explored.
> 
> Fixes: d5b892fd607a ("bpf: make infinite loop detection in is_state_visited() exact")
> Reported-by: Eduard Zingerman <eddyz87@gmail.com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

...

[PATCH bpf v2 2/2] selftests/bpf: Cover frame changes in bounded loops

[Kumar Kartikeya Dwivedi]

Add two finite loops whose progress is represented only by changing the
frame number of a stack pointer. Each loop first reads zero from the
caller's stack, switches to the same offset in the callee's stack, and
exits after reading one on its next iteration.

Keep the pointer in a register in one test and spill it in the other to
exercise both regsafe() and stacksafe() during exact state comparison.
Force frequent checkpoints so the tests exercise infinite-loop detection,
and check that both programs return one when run.

Without the frameno comparison in regs_exact(), both programs are
rejected with an "infinite loop detected" diagnostic instead of loading
successfully.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/progs/verifier_loops1.c     | 70 +++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_loops1.c b/tools/testing/selftests/bpf/progs/verifier_loops1.c
index d248ce877f14..ada7e1105a95 100644
--- a/tools/testing/selftests/bpf/progs/verifier_loops1.c
+++ b/tools/testing/selftests/bpf/progs/verifier_loops1.c
@@ -303,4 +303,74 @@ __naked void maybe_exit_scc_bug1(void)
 	::: __clobber_all);
 }
 
+/*
+ * The loop reads zero from the caller's stack on its first iteration and
+ * one from the callee's stack on its second iteration. At the loop header,
+ * only the frame number of the pointer in r1 changes.
+ */
+static __naked __noinline __used
+void loop_stack_frames_reg(void)
+{
+	asm volatile (
+	"*(u64 *)(r10 - 8) = 1;"
+	"1: r0 = *(u64 *)(r1 + 0);"
+	"if r0 != 0 goto 2f;"
+	"r1 = r10;"
+	"r1 += -8;"
+	"goto 1b;"
+	"2: exit;"
+	::: __clobber_all);
+}
+
+SEC("xdp")
+__description("bounded loop changing stack frame in a register")
+__success __retval(1)
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void bounded_loop_stack_frames_reg(void)
+{
+	asm volatile (
+	"*(u64 *)(r10 - 8) = 0;"
+	"r1 = r10;"
+	"r1 += -8;"
+	"call loop_stack_frames_reg;"
+	"exit;"
+	::: __clobber_all);
+}
+
+/*
+ * Exercise the same frame change through a spilled pointer. All registers
+ * at the loop header are identical, while fp-16 points to a different frame.
+ */
+static __naked __noinline __used
+void loop_stack_frames_spill(void)
+{
+	asm volatile (
+	"*(u64 *)(r10 - 8) = 1;"
+	"*(u64 *)(r10 - 16) = r1;"
+	"1: r1 = *(u64 *)(r10 - 16);"
+	"r0 = *(u64 *)(r1 + 0);"
+	"if r0 != 0 goto 2f;"
+	"r1 = r10;"
+	"r1 += -8;"
+	"*(u64 *)(r10 - 16) = r1;"
+	"goto 1b;"
+	"2: exit;"
+	::: __clobber_all);
+}
+
+SEC("xdp")
+__description("bounded loop changing stack frame in a spill")
+__success __retval(1)
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void bounded_loop_stack_frames_spill(void)
+{
+	asm volatile (
+	"*(u64 *)(r10 - 8) = 0;"
+	"r1 = r10;"
+	"r1 += -8;"
+	"call loop_stack_frames_spill;"
+	"exit;"
+	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.53.0

Re: [PATCH bpf v2 2/2] selftests/bpf: Cover frame changes in bounded loops

[Eduard Zingerman]

On Fri, 2026-09-18 at 03:13 +0200, Kumar Kartikeya Dwivedi wrote:
> Add two finite loops whose progress is represented only by changing the
> frame number of a stack pointer. Each loop first reads zero from the
> caller's stack, switches to the same offset in the callee's stack, and
> exits after reading one on its next iteration.
> 
> Keep the pointer in a register in one test and spill it in the other to
> exercise both regsafe() and stacksafe() during exact state comparison.
> Force frequent checkpoints so the tests exercise infinite-loop detection,
> and check that both programs return one when run.
> 
> Without the frameno comparison in regs_exact(), both programs are
> rejected with an "infinite loop detected" diagnostic instead of loading
> successfully.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Tested-by: Eduard Zingerman <eddyz87@gmail.com>

>  .../selftests/bpf/progs/verifier_loops1.c     | 70 +++++++++++++++++++
>  1 file changed, 70 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/verifier_loops1.c b/tools/testing/selftests/bpf/progs/verifier_loops1.c
> index d248ce877f14..ada7e1105a95 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_loops1.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_loops1.c
> @@ -303,4 +303,74 @@ __naked void maybe_exit_scc_bug1(void)
>  	::: __clobber_all);
>  }
>  
> +/*
> + * The loop reads zero from the caller's stack on its first iteration and
> + * one from the callee's stack on its second iteration. At the loop header,
> + * only the frame number of the pointer in r1 changes.
> + */
> +static __naked __noinline __used
> +void loop_stack_frames_reg(void)
> +{
> +	asm volatile (
> +	"*(u64 *)(r10 - 8) = 1;"
> +	"1: r0 = *(u64 *)(r1 + 0);"

Nit: please use formatting as below:

  "1:"
	"r0 = *(u64 *)(r1 + 0);"

> +	"if r0 != 0 goto 2f;"
> +	"r1 = r10;"
> +	"r1 += -8;"
> +	"goto 1b;"
> +	"2: exit;"
> +	::: __clobber_all);
> +}
> +
> +SEC("xdp")
> +__description("bounded loop changing stack frame in a register")
> +__success __retval(1)
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void bounded_loop_stack_frames_reg(void)
> +{
> +	asm volatile (
> +	"*(u64 *)(r10 - 8) = 0;"
> +	"r1 = r10;"
> +	"r1 += -8;"
> +	"call loop_stack_frames_reg;"
> +	"exit;"
> +	::: __clobber_all);
> +}

--- 8< ---

> +/*
> + * Exercise the same frame change through a spilled pointer. All registers
> + * at the loop header are identical, while fp-16 points to a different frame.
> + */
> +static __naked __noinline __used
> +void loop_stack_frames_spill(void)
> +{
> +	asm volatile (
> +	"*(u64 *)(r10 - 8) = 1;"
> +	"*(u64 *)(r10 - 16) = r1;"
> +	"1: r1 = *(u64 *)(r10 - 16);"
> +	"r0 = *(u64 *)(r1 + 0);"
> +	"if r0 != 0 goto 2f;"
> +	"r1 = r10;"
> +	"r1 += -8;"
> +	"*(u64 *)(r10 - 16) = r1;"
> +	"goto 1b;"
> +	"2: exit;"
> +	::: __clobber_all);
> +}
> +
> +SEC("xdp")
> +__description("bounded loop changing stack frame in a spill")
> +__success __retval(1)
> +__flag(BPF_F_TEST_STATE_FREQ)
> +__naked void bounded_loop_stack_frames_spill(void)
> +{
> +	asm volatile (
> +	"*(u64 *)(r10 - 8) = 0;"
> +	"r1 = r10;"
> +	"r1 += -8;"
> +	"call loop_stack_frames_spill;"
> +	"exit;"
> +	::: __clobber_all);
> +}
> +
>  char _license[] SEC("license") = "GPL";

I think this test is redundant, existing tests should already cover
the chain stack_safe -> regsafe.

--- >8 ---