On Tue, Jul 19, 2022 at 09:34:13PM +0300, Dan Carpenter wrote:
> On Tue, Jul 19, 2022 at 10:26:40AM -0700, Martin KaFai Lau wrote:
> > On Tue, Jul 19, 2022 at 12:49:34PM +0300, Dan Carpenter wrote:
> > > The code here is supposed to take a signed int and store it in a
> > > signed long long. Unfortunately, the way that the type promotion works
> > > with this conditional statement is that it takes a signed int, type
> > > promotes it to a __u32, and then stores that as a signed long long.
> > > The result is never negative.
> > >
> > > Fixes: d90ec262b35b ("libbpf: Add enum64 support for btf_dump")
> > > Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > > ---
> > > tools/lib/bpf/btf_dump.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > > index 400e84fd0578..627edb5bb6de 100644
> > > --- a/tools/lib/bpf/btf_dump.c
> > > +++ b/tools/lib/bpf/btf_dump.c
> > > @@ -2045,7 +2045,7 @@ static int btf_dump_get_enum_value(struct btf_dump *d,
> > > *value = *(__s64 *)data;
> > > return 0;
> > > case 4:
> > > - *value = is_signed ? *(__s32 *)data : *(__u32 *)data;
> > > + *value = is_signed ? (__s64)*(__s32 *)data : *(__u32 *)data;
> > Only case 4 has issues and what does the standard say ?
> >
>
> It looks weird, doesn't it?
>
> Yes. Everything smaller than int gets type promoted to int so the sign
> is extended properly. The only thing larger than s/u32 is s/u64 which
> is already the right size.
Ah. tricky.
>
> > Do you have a sample dump to debug this that can be pasted in the commit log?
>
> This is from static analysis, but I made a little test program just to
> test it before I sent the patch:
>
> #include <stdio.h>
>
> int main(void)
> {
> unsigned long long src = -1ULL;
> signed long long dst1, dst2;
> int is_signed = 1;
>
> dst1 = is_signed ? *(int *)&src : *(unsigned int *)0;
> dst2 = is_signed ? (signed long long)*(int *)&src : *(unsigned int *)0;
>
> printf("%lld\n", dst1);
> printf("%lld\n", dst2);
>
> return 0;
> }
Thanks for the demo.
Acked-by: Martin KaFai Lau <kafai@xxxxxx>
