linux-kernel - Re: [PATCH] libbpf: fix warnings "'pad_type' 'pad_bits' 'new

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Date:   Fri, 28 Jul 2023 10:10:21 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Yonghong Song <yonghong.song@...ux.dev>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        Xiangyu Chen <xiangyu.chen@....windriver.com>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] libbpf: fix warnings "'pad_type' 'pad_bits' 'new_off'
 may be used uninitialized"

On Thu, Jul 27, 2023 at 11:23:21AM -0700, Yonghong Song wrote:
> 
> 
> On 7/27/23 4:01 AM, Jiri Olsa wrote:
> > On Thu, Jul 27, 2023 at 04:25:36PM +0800, Xiangyu Chen wrote:
> > > From: Xiangyu Chen <xiangyu.chen@...driver.com>
> > > 
> > > When turn on the yocto DEBUG_BUILD flag, the build options for gcc would enable maybe-uninitialized,
> > > and following warnings would be reported as below:
> > 
> > curious, what's the gcc version? I can't reproduce that,
> > and we already have all warnings enabled:
> > 
> >    CFLAGS += -Werror -Wall
> > 
> > they seem like false warnings also, because ARRAY_SIZE(pads)
> > will be always > 0
> 
> Agree. This definitely a false positive.
> In kernel top Makefile, we have
> 
> # Enabled with W=2, disabled by default as noisy
> ifdef CONFIG_CC_IS_GCC
> KBUILD_CFLAGS += -Wno-maybe-uninitialized
> endif
> 
> That means gcc -maybe-uninitialized is very noisy.

nice, I think we should do the same then ;-)
bt not sure how to do the gcc check though

jirka

> 
> > 
> > jirka
> > 
> > > 
> > > | btf_dump.c: In function 'btf_dump_emit_bit_padding':
> > > | btf_dump.c:916:4: error: 'pad_type' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > |   916 |    btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type,
> > > |       |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > |   917 |      in_bitfield ? new_off - cur_off : 0);
> > > |       |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > | btf_dump.c:929:6: error: 'pad_bits' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > |   929 |   if (bits == pad_bits) {
> > > |       |      ^
> > > | btf_dump.c:913:28: error: 'new_off' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > |   913 |       (new_off == next_off && roundup(cur_off, next_align * 8) != new_off) ||
> > > |       |       ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > |   HOSTLD  scripts/mod/modpost
> > > 
> > > Signed-off-by: Xiangyu Chen <xiangyu.chen@...driver.com>
> > > ---
> > >   tools/lib/bpf/btf_dump.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > > index 4d9f30bf7f01..79923c3b8777 100644
> > > --- a/tools/lib/bpf/btf_dump.c
> > > +++ b/tools/lib/bpf/btf_dump.c
> > > @@ -867,8 +867,8 @@ static void btf_dump_emit_bit_padding(const struct btf_dump *d,
> > >   	} pads[] = {
> > >   		{"long", d->ptr_sz * 8}, {"int", 32}, {"short", 16}, {"char", 8}
> > >   	};
> > > -	int new_off, pad_bits, bits, i;
> > > -	const char *pad_type;
> > > +	int new_off = 0, pad_bits = 0, bits, i;
> > > +	const char *pad_type = NULL;
> > >   	if (cur_off >= next_off)
> > >   		return; /* no gap */
> > > -- 
> > > 2.34.1
> > > 
> > > 
> >