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? 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. > 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 > > >
