On 6/15/18 7:24 AM, Arnaldo Carvalho de Melo wrote:
Em Thu, Jun 14, 2018 at 09:56:43PM -0700, Yonghong Song escreveu:
I really want to get rid of this option as well. To make pahole work
with the default default format, I need to add bpf support to
libdwfl in elfutils repo. I will work on that.
Right, I haven't looked into detail, but perhaps we can do like we do in
tools/perf/ where we add a feature test to check if some function is
present in a library (elfutils even) and if so, use it, otherwise, use a
copy that we carry in pahole.git.
For instance:
tools/perf/util/symbol-elf.c
#ifndef HAVE_ELF_GETPHDRNUM_SUPPORT
static int elf_getphdrnum(Elf *elf, size_t *dst)
{
GElf_Ehdr gehdr;
GElf_Ehdr *ehdr;
ehdr = gelf_getehdr(elf, &gehdr);
if (!ehdr)
return -1;
*dst = ehdr->e_phnum;
return 0;
}
#endif
And we have a feature test to check if that is present, simple one, if
that builds and links, we have it, then the tools build Makefile magic
will end up defining HAVE_ELF_GETPHDRNUM_SUPPORT and our copy doesn't
get included, using what is in elfutils:
[acme@jouet perf]$ cat tools/build/feature/test-libelf-getphdrnum.c
// SPDX-License-Identifier: GPL-2.0
#include <libelf.h>
int main(void)
{
size_t dst;
return elf_getphdrnum(0, &dst);
}
[acme@jouet perf]$
[acme@jouet perf]$ grep elf /tmp/build/perf/FEATURE-DUMP
feature-libelf=1
feature-libelf-getphdrnum=1
feature-libelf-gelf_getnote=1
feature-libelf-getshdrstrndx=1
feature-libelf-mmap=1
[acme@jouet perf]$
This way a new pahole version won't get to wait till places where it
gets built have these new functions and we stop using it as soon as the
library get it.
Agreed that later on we can use feature testing to check pahole output
is good or not without dwarfris option. If it is good, compilation does
not need this option and we could deprecate dwarfris option and
eventually remove it once BPF support is in all major elf libraries.
BTW, I have pushed the following commit
https://reviews.llvm.org/rL334839
to clang so now you can compile with attribute dwarfris directly with
clang -tartget bpf.
-bash-4.2$ cat t.c
struct tt {
int a;
char b;
int c;
};
int test(struct tt *a) {
return a->a;
}
-bash-4.2$ clang -target bpf -O2 -g -c -Xclang -target-feature -Xclang
+dwarfris t.c
-bash-4.2$ llvm-objdump -S -d t.o
t.o: file format ELF64-BPF
Disassembly of section .text:
test:
; int test(struct tt *a) {
0: 61 10 00 00 00 00 00 00 r0 = *(u32 *)(r1 + 0)
; return a->a;
1: 95 00 00 00 00 00 00 00 exit
-bash-4.2$ pahole t.o
struct tt {
int a; /* 0 4 */
char b; /* 4 1 */
/* XXX 3 bytes hole, try to pack */
int c; /* 8 4 */
/* size: 12, cachelines: 1, members: 3 */
/* sum members: 9, holes: 1, sum holes: 3 */
/* last cacheline: 12 bytes */
};
-bash-4.2$
