Re: [PATCH bpf-next v2 1/2] libbpf: Fix sign extension of unaligned enums in btf_dump_get_enum_value()

[David Laight <david.laight.linux@gmail.com>]

On Sun, 15 Nov 2026 08:49:22 +0000
Hungyu Lin <dennylin0707@gmail.com> wrote:

> btf_dump_get_enum_value() reads unaligned enum values through
> btf_dump_get_bitfield_value() with a zero bit size. That makes
> right_shift_bits 64 in
> 
> 	*value = (num << left_shift_bits) >> right_shift_bits;
> 
> which is undefined behavior for a __u64 on both endians. The path also
> bypasses the signed loads used for aligned data, so a negative value of
> a signed enum can lose its enumerator name when the data is unaligned.
> 
> Fix this by copying unaligned enum data into an aligned buffer, as
> btf_dump_int_data() does, and reading it with the same loads as aligned
> data.
> 
> Fixes: 920d16af9b42 ("libbpf: BTF dumper support for typed data")
> Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
> ---
>  tools/lib/bpf/btf_dump.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> index bbe86928b3ec..a80762a70cd0 100644
> --- a/tools/lib/bpf/btf_dump.c
> +++ b/tools/lib/bpf/btf_dump.c
> @@ -2329,16 +2329,11 @@ static int btf_dump_get_enum_value(struct btf_dump *d,
>  				   __s64 *value)
>  {
>  	bool is_signed = btf_kflag(t);
> +	char buf[8] __attribute__((aligned(8)));
>  
> -	if (!ptr_is_aligned(d->btf, id, data)) {
> -		__u64 val;
> -		int err;
> -
> -		err = btf_dump_get_bitfield_value(d, t, data, 0, 0, &val);
> -		if (err)
> -			return err;
> -		*value = (__s64)val;
> -		return 0;
> +	if (t->size <= sizeof(buf) && !ptr_is_aligned(d->btf, id, data)) {

Do the alignment check first, you don't expect the size check to fail.

David

> +		memcpy(buf, data, t->size);
> +		data = buf;
>  	}
>  
>  	switch (t->size) {