On Thu, May 25, 2023 at 4:40 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, May 25, 2023 at 2:51 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >
> > On 5/25/23 7:19 PM, Andrii Nakryiko wrote:
> > > On Wed, May 24, 2023 at 8:18 PM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >> On Wed, May 24, 2023 at 02:02:41PM -0700, Andrii Nakryiko wrote:
> > >>>
> > >>> And there were a bunch of other similar changes. Please take a thorough
> > >>> look and suggest more changes or which changes to drop. I'm not married
> > >>> to any of them, it just felt like a good improvement.
> > >>
> > >> Agree that current layout sucks, but ...
> > >>
> > >>> include/uapi/linux/bpf.h | 235 +++++++++++++++++++++++++++------
> > >>> kernel/bpf/syscall.c | 40 +++---
> > >>> tools/include/uapi/linux/bpf.h | 235 +++++++++++++++++++++++++++------
> > >>> 3 files changed, 405 insertions(+), 105 deletions(-)
> > >>
> > >> ... the diff makes it worse. The diffstat for "nop" change is a red flag.
> > >
> > > Only 100 lines are a real "nop" change to copy/paste existing fields
> > > that are in unnamed fields. The rest is a value add.
> > >
> > > I don't think the deal is in stats, though, right?
> > >
> > >>> + /*
> > >>> + * LEGACY anonymous substructs, for backwards compatibility.
> > >>> + * Each of the below anonymous substructs are ABI compatible with one
> > >>> + * of the above named substructs. Please use named substructs.
> > >>> + */
> > >>
> > >> All of them cannot be removed. This bagage will be a forever eyesore.
> > >> Currently it's not pretty. The diffs make uapi file just ugly.
> > >> Especially considering how 'named' and 'legacy' will start diverging.
> > >
> > > We have to allow "divergence" (only in the sense that new fields only
> > > go into named substructs, but the existing fields stay fixed, of
> > > course), to avoid more naming conflicts. If that wasn't the case,
> > > using struct_group() macro could have been used to avoid a copy/paste
> > > of those anonymous field/struct copies.
> > >
> > > So I'm not happy about those 100 lines copy paste of fixed fields
> > > either, but at least that would get us out of the current global
> > > naming namespace for PROG_LOAD, MAP_CREATE, BTF_LOAD, etc.
> > >
> > >> New commands are thankfully named. We've learned the lesson,
> > >
> > > Unfortunately, the problem is that unnamed commands are the ones that
> > > are most likely to keep evolving.
> > >
> > >> but prior mistake is unfixable. We have to live with it.
> > >
> > > Ok, too bad, but it's fine. It was worth a try.
> > >
> > > I tried to come up with something like struct_group() approach to
> > > minimize code changes in UAPI header, but we have a more complicated
> > > situation where part of struct has to be both anonymous and named,
> > > while another part (newly added fields) should go only to named parts.
> > > And that doesn't seem to be possible to support with a macro,
> > > unfortunately.
> >
> > Nice idea on the struct_group()-like approach, but agree that this is
> > going to be tough given we need to divert anonymous and named parts as
> > you mention. One other wild thought ... we remove the bpf_attr entirely
> > from the uapi header, and have a kernel/bpf/bpf.cmd description and
> > generate the bpf_attr into a uapi header via script which the main header
> > can include. Kind of similar to the suggestion, but more flexible than
> > macro magic. We also have things like syscall table header generated via
> > script.. so it wouldn't be first. Still doesn't remove the eyesore, just
> > packages it differently. ;/
>
> There are two more ways, neither is that pretty. But I'll just outline
> them here for completeness.
>
> First, we can define about 6 variants (one for each command with anon
> field) of macro with different numbers of arguments, one for each
> existing field. Replace all semicolons with commas and do something
> like this (we can prettify the below some more, I didn't want to waste
> too much time on this demo):
>
> #define __bpf_cmd4(type, f1, f2, f3, f4, new_fields...) \
> struct { \
> f1; f2; f3; f4; \
> }; \
> struct type { \
> f1; f2; f3; f4; \
> new_fields \
> }
>
> /* BPF_OBJ_PIN command */
> __bpf_cmd4(bpf_obj_pin_attr,
> __aligned_u64 pathname,
> __u32 bpf_fd,
> __u32 file_flags,
> /* Same as dirfd in openat() syscall; see openat(2)
> * manpage for details of path FD and pathname semantics;
> * path_fd should accompanied by BPF_F_PATH_FD flag set in
> * file_flags field, otherwise it should be set to zero;
> * if BPF_F_PATH_FD flag is not set, AT_FDCWD is assumed.
> */
> __s32 path_fd,
> __u32 token_fd;
> ) obj_pin;
>
> Note that I also added `__u32 token_fd;` as a demonstration how we can
> new fields, and that new fields will have proper semicolons at the
> end. The largest command (BPF_PROG_LOAD) will need 28 arg variant, but
> that can be fit in few lines pretty cleanly, if the overall approach
> would be deemed acceptable.
>
> This approach also has a slight downside that we can rename fields
> (e.g. for BPF_BTF_LOAD command). We still can split out dedicated new
> named structs. So too big of a deal.
>
>
> Second approach. If it's mostly about aesthetics, then we can add
> include/uapi/linux/bpf_legacy.h, where we put all these unnamed fields
> and structs in one stashed away place, and then in original
> include/uapi/linux/bpf.h header we just
>
> union bpf_attr {
> ... named structs and fields go here ...
>
> /* include backwards compat legacy anon fields/structs */
> #include "bpf_legacy.h"
> };
>
> This way this eyesore will be somewhat hidden away (but still lookup-able).
>
>
> Curious if any of the above is more palatable?
Frankly I don't like either Daniel's .cmd idea or these two "aesthetics".
We just need new *_token_fd fields in several structures.
imo adding several such fields with different prefixes are cleaner
than revamping the whole thing.
