Re: [PATCH] libbpf: fix warnings "'pad_type' 'pad_bits' 'new_off' may be used uninitialized"

[Yonghong Song]

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@xxxxxxxxxxxxx>
> >
> > 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.

> 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@xxxxxxxxxxxxx>
> > ---
> >   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