[PATCH bpf v2] bpf: fix argument type in bpf_loop documentation

[PATCH bpf v2] bpf: fix argument type in bpf_loop documentation

[Matteo Croce]

From: Matteo Croce <teknoraver@meta.com>

The `index` argument to bpf_loop() is threaded as an u64.
This lead in a subtle verifier denial where clang cloned the argument
in another register[1].

[1] https://github.com/systemd/systemd/pull/34650#issuecomment-2401092895

Signed-off-by: Matteo Croce <teknoraver@meta.com>
---
 include/uapi/linux/bpf.h       | 2 +-
 kernel/bpf/verifier.c          | 2 +-
 tools/include/uapi/linux/bpf.h | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 8ab4d8184b9d..874af0186fe8 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5371,7 +5371,7 @@ union bpf_attr {
  *		Currently, the **flags** must be 0. Currently, nr_loops is
  *		limited to 1 << 23 (~8 million) loops.
  *
- *		long (\*callback_fn)(u32 index, void \*ctx);
+ *		long (\*callback_fn)(u64 index, void \*ctx);
  *
  *		where **index** is the current index in the loop. The index
  *		is zero-indexed.
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7d9b38ffd220..cfc62e0776bf 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9917,7 +9917,7 @@ static int set_loop_callback_state(struct bpf_verifier_env *env,
 {
 	/* bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx,
 	 *	    u64 flags);
-	 * callback_fn(u32 index, void *callback_ctx);
+	 * callback_fn(u64 index, void *callback_ctx);
 	 */
 	callee->regs[BPF_REG_1].type = SCALAR_VALUE;
 	callee->regs[BPF_REG_2] = caller->regs[BPF_REG_3];
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 7610883c8191..5937c39069ba 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5371,7 +5371,7 @@ union bpf_attr {
  *		Currently, the **flags** must be 0. Currently, nr_loops is
  *		limited to 1 << 23 (~8 million) loops.
  *
- *		long (\*callback_fn)(u32 index, void \*ctx);
+ *		long (\*callback_fn)(u64 index, void \*ctx);
  *
  *		where **index** is the current index in the loop. The index
  *		is zero-indexed.
-- 
2.46.0

Re: [PATCH bpf v2] bpf: fix argument type in bpf_loop documentation

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu, 10 Oct 2024 04:56:52 +0100 you wrote:
> From: Matteo Croce <teknoraver@meta.com>
> 
> The `index` argument to bpf_loop() is threaded as an u64.
> This lead in a subtle verifier denial where clang cloned the argument
> in another register[1].
> 
> [1] https://github.com/systemd/systemd/pull/34650#issuecomment-2401092895
> 
> [...]

Here is the summary with links:
  - [bpf,v2] bpf: fix argument type in bpf_loop documentation
    https://git.kernel.org/bpf/bpf-next/c/5bd48a3a14df

You are awesome, thank you!
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