On Wed, Mar 22, 2023 at 12:47:54PM -0700, JP Kobryn wrote:
> This patch changes the return types of bpf_map_ops functions to long, where
> previously int was returned. Using long allows for bpf programs to maintain
> the sign bit in the absence of sign extension during situations where
> inlined bpf helper funcs make calls to the bpf_map_ops funcs and a negative
> error is returned.
>
> The definitions of the helper funcs are generated from comments in the bpf
> uapi header at `include/uapi/linux/bpf.h`. The return type of these
> helpers was previously changed from int to long in commit bdb7b79b4ce8. For
> any case where one of the map helpers call the bpf_map_ops funcs that are
> still returning 32-bit int, a compiler might not include sign extension
> instructions to properly convert the 32-bit negative value a 64-bit
> negative value.
>
> For example:
> bpf assembly excerpt of an inlined helper calling a kernel function and
> checking for a specific error:
>
> ; err = bpf_map_update_elem(&mymap, &key, &val, BPF_NOEXIST);
> ...
> 46: call 0xffffffffe103291c ; htab_map_update_elem
> ; if (err && err != -EEXIST) {
> 4b: cmp $0xffffffffffffffef,%rax ; cmp -EEXIST,%rax
>
> kernel function assembly excerpt of return value from
> `htab_map_update_elem` returning 32-bit int:
>
> movl $0xffffffef, %r9d
> ...
> movl %r9d, %eax
>
> ...results in the comparison:
> cmp $0xffffffffffffffef, $0x00000000ffffffef
>
> Fixes: bdb7b79b4ce8 (bpf: Switch most helper return values from 32-bit int
> to 64-bit long)
please don't break Fixes tag into multiple lines.
Also keep "".
I fixed it this time while applying.
