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...
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. The pahole patch uses --update-section if
the section exists rather than writing out a new ELF file. Thoughts?
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
> > > > > >
