Re: [PATCH 1/1] btf_encoder: Generate a new .BTF section even if one exists

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On Thu, Dec 1, 2022 at 11:56 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Nov 30, 2022 at 4:21 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
> >
> > On Wed, Nov 30, 2022 at 2:59 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Mon, Nov 21, 2022 at 4:00 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
> > > >
> > > > LLD generates a zero-length .BTF section (BFD doesn't generate this
> > > > section). It shares the same address as .BTF_ids (or any section below
> > > > it). E.g.:
> > > >
> > > >   [24] .BTF              PROGBITS        ffffffff825a1900 17a1900 000000
> > > >   [25] .BTF_ids          PROGBITS        ffffffff825a1900 17a1900 000634
> > > >
> > > > Writing new data to that section doesn't adjust the addresses of
> > > > following sections. As a result, the "-J" flag produces a corrupted
> > > > file, causing further commands to fail.
> > > >
> > > > Instead of trying to adjust everything, just add a new section with the
> > > > .BTF data and adjust the name of the original .BTF section. (We can't
> > > > remove the old .BTF section because it has variables that are referenced
> > > > elsewhere.)
> > > >
> > >
> > > Have you tried llvm-objcopy --update-section instead? Doesn't it work?
> > >
> > I gave it a quick try and it fails for me with this:
> >
> > llvm-objcopy: error: '.tmp_vmlinux.btf': cannot fit data of size
> > 4470718 into section '.BTF' with size 0 that is part of a segment
>
> .BTF shouldn't be allocatable section, when added by pahole. I think
> this is the problem. Can you confirm that that zero-sized .BTF is
> marked as allocated and is put into one of ELF segments? Can we fix
> that instead?
>
I think it does:

[24] .BTF              PROGBITS        ffffffff825a1900 17a1900 000000
00  WA  0   0  1

Fangrui mentioned something similar to this in a previous message:

  https://lore.kernel.org/dwarves/20210317232657.mdnsuoqx6nbddjgt@xxxxxxxxxx/T/#u

> Also, more generally, newer paholes (not that new anymore, it's been a
> supported feature for a while) support emitting BTF as raw binary
> files, instead of embedding them into ELF. I think this is a nicer and
> simpler option and we should switch link-vmlinux.sh to use that
> instead, if pahole is new enough.
>
> Hopefully eventually we can get rid of all the old pahole version
> cruft, but for now it's inevitable to support both modes, of course.
>

Ah technical debt! :-)

-bw

> > btf_encoder__write_elf: failed to add .BTF section to '.tmp_vmlinux.btf': 2!
> > Failed to encode BTF
> >
> > -bw
> >
> > > > Link: https://lore.kernel.org/dwarves/20210317232657.mdnsuoqx6nbddjgt@xxxxxxxxxx/
> > > > Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> > > > Cc: Fangrui Song <maskray@xxxxxxxxxx>
> > > > Cc: dwarves@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> > > > ---
> > > >  btf_encoder.c | 88 +++++++++++++++++++++++++++++++--------------------
> > > >  1 file changed, 54 insertions(+), 34 deletions(-)
> > > >
> > > > diff --git a/btf_encoder.c b/btf_encoder.c
> > > > index a5fa04a84ee2..bd1ce63e992c 100644
> > > > --- a/btf_encoder.c
> > > > +++ b/btf_encoder.c
> > > > @@ -1039,6 +1039,9 @@ static int btf_encoder__write_elf(struct btf_encoder *encoder)
> > > >         Elf_Data *btf_data = NULL;
> > > >         Elf_Scn *scn = NULL;
> > > >         Elf *elf = NULL;
> > > > +       const char *llvm_objcopy;
> > > > +       char tmp_fn[PATH_MAX];
> > > > +       char cmd[PATH_MAX * 2];
> > > >         const void *raw_btf_data;
> > > >         uint32_t raw_btf_size;
> > > >         int fd, err = -1;
> > > > @@ -1081,42 +1084,58 @@ static int btf_encoder__write_elf(struct btf_encoder *encoder)
> > > >
> > > >         raw_btf_data = btf__raw_data(btf, &raw_btf_size);
> > > >
> > > > +       llvm_objcopy = getenv("LLVM_OBJCOPY");
> > > > +       if (!llvm_objcopy)
> > > > +               llvm_objcopy = "llvm-objcopy";
> > > > +
> > > > +       /* Use objcopy to add a .BTF section */
> > > > +       snprintf(tmp_fn, sizeof(tmp_fn), "%s.btf", filename);
> > > > +       close(fd);
> > > > +       fd = creat(tmp_fn, S_IRUSR | S_IWUSR);
> > > > +       if (fd == -1) {
> > > > +               fprintf(stderr, "%s: open(%s) failed!\n", __func__,
> > > > +                       tmp_fn);
> > > > +               goto out;
> > > > +       }
> > > > +
> > > > +       if (write(fd, raw_btf_data, raw_btf_size) != raw_btf_size) {
> > > > +               fprintf(stderr, "%s: write of %d bytes to '%s' failed: %d!\n",
> > > > +                       __func__, raw_btf_size, tmp_fn, errno);
> > > > +               goto unlink;
> > > > +       }
> > > > +
> > > >         if (btf_data) {
> > > > -               /* Existing .BTF section found */
> > > > -               btf_data->d_buf = (void *)raw_btf_data;
> > > > -               btf_data->d_size = raw_btf_size;
> > > > -               elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
> > > > -
> > > > -               if (elf_update(elf, ELF_C_NULL) >= 0 &&
> > > > -                   elf_update(elf, ELF_C_WRITE) >= 0)
> > > > -                       err = 0;
> > > > -               else
> > > > -                       elf_error("elf_update failed");
> > > > -       } else {
> > > > -               const char *llvm_objcopy;
> > > > -               char tmp_fn[PATH_MAX];
> > > > -               char cmd[PATH_MAX * 2];
> > > > -
> > > > -               llvm_objcopy = getenv("LLVM_OBJCOPY");
> > > > -               if (!llvm_objcopy)
> > > > -                       llvm_objcopy = "llvm-objcopy";
> > > > -
> > > > -               /* Use objcopy to add a .BTF section */
> > > > -               snprintf(tmp_fn, sizeof(tmp_fn), "%s.btf", filename);
> > > > -               close(fd);
> > > > -               fd = creat(tmp_fn, S_IRUSR | S_IWUSR);
> > > > -               if (fd == -1) {
> > > > -                       fprintf(stderr, "%s: open(%s) failed!\n", __func__,
> > > > -                               tmp_fn);
> > > > -                       goto out;
> > > > -               }
> > > > -
> > > > -               if (write(fd, raw_btf_data, raw_btf_size) != raw_btf_size) {
> > > > -                       fprintf(stderr, "%s: write of %d bytes to '%s' failed: %d!\n",
> > > > -                               __func__, raw_btf_size, tmp_fn, errno);
> > > > +               /*
> > > > +                * Existing .BTF section found. LLD creates a zero-sized .BTF
> > > > +                * section. Adding data to that section doesn't change the
> > > > +                * addresses of the other sections, causing an overwriting of
> > > > +                * data. These commands are a bit convoluted, but they will add
> > > > +                * a new .BTF section with the proper size. Note though that
> > > > +                * the __start_btf and __stop_btf variables aren't affected by
> > > > +                * this change, but then they aren't added when using
> > > > +                * "--add-section" either.
> > > > +                */
> > > > +               snprintf(cmd, sizeof(cmd),
> > > > +                        "%s --add-section .BTF.new=%s "
> > > > +                        "--rename-section .BTF=.BTF.old %s",
> > > > +                        llvm_objcopy, tmp_fn, filename);
> > > > +               if (system(cmd)) {
> > > > +                       fprintf(stderr, "%s: failed to add .BTF section to '%s': %d!\n",
> > > > +                               __func__, filename, errno);
> > > >                         goto unlink;
> > > >                 }
> > > >
> > > > +               snprintf(cmd, sizeof(cmd),
> > > > +                        "%s --rename-section .BTF.new=.BTF %s",
> > > > +                        llvm_objcopy, filename);
> > > > +               if (system(cmd)) {
> > > > +                       fprintf(stderr, "%s: failed to rename .BTF section to '%s': %d!\n",
> > > > +                               __func__, filename, errno);
> > > > +                       goto unlink;
> > > > +               }
> > > > +
> > > > +               err = 0;
> > > > +       } else {
> > > >                 snprintf(cmd, sizeof(cmd), "%s --add-section .BTF=%s %s",
> > > >                          llvm_objcopy, tmp_fn, filename);
> > > >                 if (system(cmd)) {
> > > > @@ -1126,10 +1145,11 @@ static int btf_encoder__write_elf(struct btf_encoder *encoder)
> > > >                 }
> > > >
> > > >                 err = 0;
> > > > -       unlink:
> > > > -               unlink(tmp_fn);
> > > >         }
> > > >
> > > > +unlink:
> > > > +       unlink(tmp_fn);
> > > > +
> > > >  out:
> > > >         if (fd != -1)
> > > >                 close(fd);
> > > > --
> > > > 2.38.1.584.g0f3c55d4c2-goog
> > > >



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