adding bpf@vger back
On Tue, Dec 6, 2022 at 12:15 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
>
> On Tue, Dec 6, 2022 at 10:38 AM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Thu, Dec 1, 2022 at 12:20 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
> > >
> > > 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
> > >
> >
> > So this allocatable .BTF section, could it be because of linker script
> > in include/asm-generic/vmlinux.lds.h? Should we add some conditions
> > there to not emit .BTF if __startt_BTF == __stop_BTF (i.e., no BTF
> > data is present) to avoid this issue in the first place?
> >
> It looks like keeping the .BTF section around is intentional:
>
> commit 65c204398928 ("bpf: Prevent .BTF section elimination")
>
> I assume that patch isn't meant if the section is zero sized...
yep, we need to keep it only if it's non-empty
>
> I was able to get a working system with two patches: one to Linux and
> one to pahole. The Linux patch specifies that the .BTF section
> shouldn't be allocatable.
That's not right, we do want this .BTF section to be allocatable,
kernel expects this content to be accessible at runtime. So Linux-side
change is wrong. Is it possible to add some conditional statement to
linker script to keep .BTF only if .BTF is non-empty?
> The pahole patch uses --update-section if
> the section exists rather than writing out a new ELF file. Thoughts?
That might be ok, because we already have dependency on llvm-objcopy.
But also it's unnecessary change if the section in not allocated,
right? Or why do we need to switch to llvm-objcopy in this case?
>
> Linux patch:
>
> diff --git a/include/asm-generic/vmlinux.lds.h
> b/include/asm-generic/vmlinux.lds.h
> index 3dc5824141cd..5bea090b736e 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -680,7 +680,7 @@
> */
> #ifdef CONFIG_DEBUG_INFO_BTF
> #define BTF \
> - .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { \
> + .BTF (INFO) : AT(ADDR(.BTF) - LOAD_OFFSET) { \
> __start_BTF = .; \
> KEEP(*(.BTF)) \
> __stop_BTF = .; \
>
> pahole patch:
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index a5fa04a84ee2..546d32aac4f1 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -1040,6 +1040,9 @@ static int btf_encoder__write_elf(struct
> btf_encoder *encoder)
> Elf_Scn *scn = NULL;
> Elf *elf = NULL;
> const void *raw_btf_data;
> + const char *llvm_objcopy;
> + char tmp_fn[PATH_MAX];
> + char cmd[PATH_MAX * 2];
> uint32_t raw_btf_size;
> int fd, err = -1;
> size_t strndx;
> @@ -1081,55 +1084,44 @@ 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");
> + snprintf(cmd, sizeof(cmd), "%s --update-section .BTF=%s %s",
> + llvm_objcopy, tmp_fn, filename);
> } 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);
> - goto unlink;
> - }
> -
> snprintf(cmd, sizeof(cmd), "%s --add-section .BTF=%s %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;
> - }
> -
> - err = 0;
> - unlink:
> - unlink(tmp_fn);
> }
>
> + if (system(cmd)) {
> + fprintf(stderr, "%s: failed to add .BTF section to '%s': %d!\n",
> + __func__, filename, errno);
> + goto unlink;
> + }
> +
> + err = 0;
> +unlink:
> + unlink(tmp_fn);
> +
> out:
> if (fd != -1)
> close(fd);
>
>
> > > 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! :-)
> >
> > Yep, it would be good to get contributions to address it ;) It's
> > better than hacks with renaming of sections, *wink wink* :)
> >
> ;-)
>
> -bw
>
> > >
> > > -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
> > > > > > >
