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Message-ID: <CAEf4Bzb6hhyyiAyyZZAA2pUZRNmfjAw_63ES8owfGvT_QXMyTw@mail.gmail.com>
Date: Tue, 28 Oct 2025 09:09:03 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
Cc: andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, daniel@...earbox.net,
martin.lau@...ux.dev, song@...nel.org, yonghong.song@...ux.dev,
john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
haoluo@...gle.com, jolsa@...nel.org, shuah@...nel.org, nathan@...nel.org,
nick.desaulniers+lkml@...il.com, morbo@...gle.com, justinstitt@...gle.com,
ameryhung@...il.com, toke@...hat.com, bpf@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
llvm@...ts.linux.dev, skhan@...uxfoundation.org, david.hunter.linux@...il.com,
khalid@...nel.org, linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v4] selftests/bpf: Change variable types for -Wsign-compare
On Mon, Oct 20, 2025 at 5:32 AM Mehdi Ben Hadj Khelifa
<mehdi.benhadjkhelifa@...il.com> wrote:
>
> 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;
>
this is wrong, i is coming from bpf_for() and is signed int
I'd suggest dropping this patch altogether, it's not helpful and
doesn't fix any real bugs.
pw-bot: cr
> bpf_for(i, 0, sch->q.qlen) {
> struct sk_buff *skb = NULL;
> --
> 2.51.1.dirty
>
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