Re: [PATCH] btf: Add support for the floating-point types

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Adding Jiri to the CC list.

Em Sun, Mar 07, 2021 at 10:50:19AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Sun, Mar 07, 2021 at 10:44:21AM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Sun, Mar 07, 2021 at 10:16:31AM -0300, Arnaldo Carvalho de Melo escreveu:
> > > Em Sat, Mar 06, 2021 at 07:16:08PM -0800, Andrii Nakryiko escreveu:
> > > > On Fri, Mar 5, 2021 at 6:22 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> > > > >
> > > > > Some BPF programs compiled on s390 fail to load, because s390
> > > > > arch-specific linux headers contain float and double types.
> > > > >
> > > > > Fix as follows:
> > > > >
> > > > > - Make DWARF loader fill base_type.float_type.
> > > > >
> > > > > - libbpf introduced support for the floating-point types in commit
> > > > >   986962fade5, so update the libbpf submodule to that version and use
> > > > >   the new btf__add_float() function in order to emit the floating-point
> > > > >   types when base_type.float_type is set.
> > > > >
> > > > > Example of the resulting entry in the vmlinux BTF:
> > > > >
> > > > >     [7164] FLOAT 'double' size=8
> > 
> > > > [PATCH dwarves] would make it a bit clearer that this is pahole patch.
> > 
> > > > But LGTM.
> >  
> > > So older versions of bpftool will fail with a .BTF section having this
> > > new float? I thought it would just skip it emitting a warning? Probably
> > > not possible as we don't have the record size encoded in a header,
> > > right?
> >  
> > > [acme@five pahole]$ bpftool btf dump file vmlinux | grep -w FLOAT
> > > [acme@five pahole]$ type pahole
> > > pahole is /home/acme/bin/pahole
> > > [acme@five pahole]$ ls -la ~/bin/pahole
> > > lrwxrwxrwx. 1 acme acme 34 Jan 29 11:00 /home/acme/bin/pahole -> /home/acme/git/pahole/build/pahole
> > > [acme@five pahole]$ pahole -J vmlinux
> > > [acme@five pahole]$ bpftool btf dump file vmlinux | grep -w FLOAT | head
> > > Error: failed to load BTF from vmlinux: Invalid argument
> > > [acme@five pahole]$
> > > 
> > > Perhaps the warning emitted by bpftool should suggest updating the tool
> > > as it found a record type it doesn't know about?
> > > 
> > > /me goes to update bpftool...
> > 
> > Works with the bpftool in bpf-next:
> > 
> > [acme@five pahole]$ bpftool btf dump file vmlinux | grep -w FLOAT | head
> > [8006] FLOAT 'double' size=8
> > [acme@five pahole]$
> 
> Applied, with this committer notes:
> 
> Committer testing:
> 
>   $ rm -rf build  # To update the libbpf git submodule
>   $ mkdir build
>   $ cd build
>   $ cmake ..
>   $ cd ..
>   $ make -C build
>   # No BTF_KIND_FLOAT before:
>   $ bpftool btf dump file vmlinux | grep -w FLOAT
>   $ type pahole
>   pahole is /home/acme/bin/pahole
>   $ ls -la ~/bin/pahole
>   lrwxrwxrwx. 1 acme acme 34 Jan 29 11:00 /home/acme/bin/pahole -> /home/acme/git/pahole/build/pahole
>   # Encode BTF:
>   $ pahole -J vmlinux
>   $ bpftool btf dump file vmlinux | grep -w FLOAT | head
>   Error: failed to load BTF from vmlinux: Invalid argument
>   $
>   # Update bpftool to what is in bpf-next, then try again:
>   $ bpftool btf dump file vmlinux | grep -w FLOAT
>   [8006] FLOAT 'double' size=8
>   $
>   # Now check that pahole works well, i.e. that the BTF loader works
>   $ pahole -F btf vmlinux -C sk_buff_head
>   struct sk_buff_head {
>         struct sk_buff *           next;                 /*     0     8 */
>         struct sk_buff *           prev;                 /*     8     8 */
>         __u32                      qlen;                 /*    16     4 */
>         spinlock_t                 lock;                 /*    20     4 */
> 
>         /* size: 24, cachelines: 1, members: 4 */
>         /* last cacheline: 24 bytes */
>   };
>   $
>   $ pahole -F btf vmlinux  | wc -l
>   122676
>   $
> 
> Now will build a kernel with this new version, reboot, then push
> publicly.

So now trying to build v5.12-rc2 with pahole supporting BTF_KIND_FLOAT:

  AS      .tmp_vmlinux.kallsyms2.S
  LD      vmlinux
  BTFIDS  vmlinux
FAILED: load BTF from vmlinux: Invalid argument
make[1]: *** [/home/acme/git/linux/Makefile:1197: vmlinux] Error 255
make[1]: Leaving directory '/home/acme/git/build/v5.12.0-rc2'
make: *** [Makefile:215: __sub-make] Error 2
[acme@five linux]$

[acme@five linux]$ egrep BTF\|DWARF  ../build/v5.12.0-rc2/.config
CONFIG_VIDEO_SONY_BTF_MPX=m
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
# CONFIG_DEBUG_INFO_DWARF4 is not set
CONFIG_DEBUG_INFO_BTF=y
CONFIG_PAHOLE_HAS_SPLIT_BTF=y
CONFIG_DEBUG_INFO_BTF_MODULES=y
[acme@five linux]$

Ideas?

- Arnaldo