netdev - [PATCH v2 bpf 08/11] samples/bpf: fix false-positive right-shift underflow warnings

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Message-ID: <20220421003152.339542-9-alobakin@pm.me>
Date:   Thu, 21 Apr 2022 00:39:41 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Alexander Lobakin <alobakin@...me>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Song Liu <songliubraving@...com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 bpf 08/11] samples/bpf: fix false-positive right-shift underflow warnings

On 32 bit systems, shifting an unsigned long by 32 positions
yields the following warning:

samples/bpf/tracex2_kern.c:60:23: warning: shift count >= width of type [-Wshift-count-overflow]
        unsigned int hi = v >> 32;
                            ^  ~~

sizeof(long) is always 8 for the BPF architecture, so this is not
correct, but the BPF samples Makefile still uses the Clang native +
LLC combo which enforces that.
Until the samples are switched to `-target bpf`, do it the usual
way: shift by 16 two times (see upper_32_bits() macro in the
kernel).

Fixes: d822a1926849 ("samples/bpf: Add counting example for kfree_skb() function calls and the write() syscall")
Fixes: 0fb1170ee68a ("bpf: BPF based latency tracing")
Fixes: f74599f7c530 ("bpf: Add tests and samples for LWT-BPF")
Acked-by: Song Liu <songliubraving@...com>
Signed-off-by: Alexander Lobakin <alobakin@...me>
---
 samples/bpf/lathist_kern.c      | 2 +-
 samples/bpf/lwt_len_hist_kern.c | 2 +-
 samples/bpf/tracex2_kern.c      | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/samples/bpf/lathist_kern.c b/samples/bpf/lathist_kern.c
index 4adfcbbe6ef4..9744ed547abe 100644
--- a/samples/bpf/lathist_kern.c
+++ b/samples/bpf/lathist_kern.c
@@ -53,7 +53,7 @@ static unsigned int log2(unsigned int v)

 static unsigned int log2l(unsigned long v)
 {
-	unsigned int hi = v >> 32;
+	unsigned int hi = (v >> 16) >> 16;

 	if (hi)
 		return log2(hi) + 32;
diff --git a/samples/bpf/lwt_len_hist_kern.c b/samples/bpf/lwt_len_hist_kern.c
index 1fa14c54963a..bf32fa04c91f 100644
--- a/samples/bpf/lwt_len_hist_kern.c
+++ b/samples/bpf/lwt_len_hist_kern.c
@@ -49,7 +49,7 @@ static unsigned int log2(unsigned int v)

 static unsigned int log2l(unsigned long v)
 {
-	unsigned int hi = v >> 32;
+	unsigned int hi = (v >> 16) >> 16;
 	if (hi)
 		return log2(hi) + 32;
 	else
diff --git a/samples/bpf/tracex2_kern.c b/samples/bpf/tracex2_kern.c
index 5bc696bac27d..6bf22056ff95 100644
--- a/samples/bpf/tracex2_kern.c
+++ b/samples/bpf/tracex2_kern.c
@@ -57,7 +57,7 @@ static unsigned int log2(unsigned int v)

 static unsigned int log2l(unsigned long v)
 {
-	unsigned int hi = v >> 32;
+	unsigned int hi = (v >> 16) >> 16;
 	if (hi)
 		return log2(hi) + 32;
 	else
--
2.36.0