Re: [PATCH] libbpf: fix sign expansion bug in btf_dump_get_enum_value()

[Dan Carpenter <dan.carpenter@oracle.com>]

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@oracle.com>
> > ---
> >  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.

> 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;
}

regards,
dan carpenter