linux-kernel - [PATCH v4] selftests/bpf: Change variable types for -Wsign-compare

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Message-ID: <20251020133156.215326-1-mehdi.benhadjkhelifa@gmail.com>
Date: Mon, 20 Oct 2025 14:31:33 +0100
From: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
To: andrii@...nel.org,
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	Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
Subject: [PATCH v4] selftests/bpf: Change variable types for -Wsign-compare

This is a follow up patch for commit 495d2d8133fd("selftests/bpf: Attempt
to build BPF programs with -Wsign-compare") from Alexei Starovoitov[1]
to be able to enable -Wsign-compare C compilation flag for clang since
-Wall doesn't add it and BPF programs are built with clang.This has the
benefit to catch problematic comparisons in future tests as quoted from
the commit message:"
  int i = -1;
  unsigned int j = 1;
  if (i < j) // this is false.

  long i = -1;
  unsigned int j = 1;
  if (i < j) // this is true.

C standard for reference:

- If either operand is unsigned long the other shall be converted to 
unsigned long.

- Otherwise, if one operand is a long int and the other unsigned int,
then if a long int can represent all the values of an unsigned int,
the unsigned int shall be converted to a long int;
otherwise both operands shall be converted to unsigned long int.

- Otherwise, if either operand is long, the other shall be 
converted to long.

- Otherwise, if either operand is unsigned, the other shall be
converted to unsigned.

Unfortunately clang's -Wsign-compare is very noisy.
It complains about (s32)a == (u32)b which is safe and doen't
have surprising behavior."

This specific patch supresses the following warnings when
-Wsign-compare is enabled:

1 warning generated.

progs/bpf_iter_bpf_percpu_array_map.c:35:16: warning: comparison of 
integers of different signs: 'int' and 'const volatile __u32' 
(aka 'const volatile unsigned int') [-Wsign-compare]
   35 |         for (i = 0; i < num_cpus; i++) {
      |                     ~ ^ ~~~~~~~~

1 warning generated.

progs/bpf_qdisc_fifo.c:93:2: warning: comparison of integers of 
different signs: 'int' and '__u32' 
(aka 'unsigned int') [-Wsign-compare]
   93 |         bpf_for(i, 0, sch->q.qlen) {
      |         ^       ~     ~~~~~~~~~~~

Should be noted that many more similar changes are still needed in order
to be able to enable the -Wsign-compare flag since -Werror is enabled and
would cause compilation of bpf selftests to fail.

[1].
Link:https://github.com/torvalds/linux/commit/495d2d8133fd1407519170a5238f455abbd9ec9b

Signed-off-by: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
---
Changelog:

Changes from v3:

-Downsized the patch as suggested by vivek yadav[2].

-Changed the commit message as suggested by Daniel Borkmann[3].

Link:https://lore.kernel.org/all/20250925103559.14876-1-mehdi.benhadjkhelifa@gmail.com/#r

Changes from v2:

-Split up the patch into a patch series as suggested by vivek

-Include only changes to variable types with no casting by my mentor
david

-Removed the -Wsign-compare in Makefile to avoid compilation errors
until adding casting for rest of comparisons.

Link:https://lore.kernel.org/bpf/20250924195731.6374-1-mehdi.benhadjkhelifa@gmail.com/T/#u

Changes from v1:

- Fix CI failed builds where it failed due to do missing .c and
.h files in my patch for working in mainline.

Link:https://lore.kernel.org/bpf/20250924162408.815137-1-mehdi.benhadjkhelifa@gmail.com/T/#u

[2]:https://lore.kernel.org/all/CABPSWR7_w3mxr74wCDEF=MYYuG2F_vMJeD-dqotc8MDmaS_FpQ@mail.gmail.com/
[3]:https://lore.kernel.org/all/5ad26663-a3cc-4bf4-9d6f-8213ac8e8ce6@iogearbox.net/
 .../testing/selftests/bpf/progs/bpf_iter_bpf_percpu_array_map.c | 2 +-
 tools/testing/selftests/bpf/progs/bpf_qdisc_fifo.c              | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_percpu_array_map.c b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_percpu_array_map.c
index 9fdea8cd4c6f..0baf00463f35 100644
--- a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_percpu_array_map.c
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_percpu_array_map.c
@@ -24,7 +24,7 @@ int dump_bpf_percpu_array_map(struct bpf_iter__bpf_map_elem *ctx)
 	__u32 *key = ctx->key;
 	void *pptr = ctx->value;
 	__u32 step;
-	int i;
+	__u32 i;
 
 	if (key == (void *)0 || pptr == (void *)0)
 		return 0;
diff --git a/tools/testing/selftests/bpf/progs/bpf_qdisc_fifo.c b/tools/testing/selftests/bpf/progs/bpf_qdisc_fifo.c
index 1de2be3e370b..7a639dcb23a9 100644
--- a/tools/testing/selftests/bpf/progs/bpf_qdisc_fifo.c
+++ b/tools/testing/selftests/bpf/progs/bpf_qdisc_fifo.c
@@ -88,7 +88,7 @@ void BPF_PROG(bpf_fifo_reset, struct Qdisc *sch)
 {
 	struct bpf_list_node *node;
 	struct skb_node *skbn;
-	int i;
+	__u32 i;
 
 	bpf_for(i, 0, sch->q.qlen) {
 		struct sk_buff *skb = NULL;
-- 
2.51.1.dirty