From: Jiri Olsa <jolsa@kernel.org>
To: Arnaldo Carvalho de Melo <acme@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andriin@fb.com>
Cc: dwarves@vger.kernel.org, netdev@vger.kernel.org,
bpf@vger.kernel.org, Yonghong Song <yhs@fb.com>,
Hao Luo <haoluo@google.com>, Martin KaFai Lau <kafai@fb.com>,
Song Liu <songliubraving@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@chromium.org>,
Joe Lawrence <joe.lawrence@redhat.com>,
Mark Wielaard <mjw@redhat.com>
Subject: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values
Date: Tue, 19 Jan 2021 23:12:19 +0100 [thread overview]
Message-ID: <20210119221220.1745061-3-jolsa@kernel.org> (raw)
In-Reply-To: <20210119221220.1745061-1-jolsa@kernel.org>
For very large ELF objects (with many sections), we could
get special value SHN_XINDEX (65535) for symbol's st_shndx.
This patch is adding code to detect the optional extended
section index table and use it to resolve symbol's section
index id needed.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
btf_encoder.c | 18 ++++++++++++++++++
elf_symtab.c | 31 ++++++++++++++++++++++++++++++-
elf_symtab.h | 1 +
3 files changed, 49 insertions(+), 1 deletion(-)
diff --git a/btf_encoder.c b/btf_encoder.c
index 5557c9efd365..2ab6815dc2b3 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -609,6 +609,24 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
/* search within symtab for percpu variables */
elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
+ struct elf_symtab *symtab = btfe->symtab;
+
+ /*
+ * Symbol's st_shndx needs to be translated with the
+ * extended section index table.
+ */
+ if (sym.st_shndx == SHN_XINDEX) {
+ Elf32_Word xndx;
+
+ if (!symtab->syms_shndx) {
+ fprintf(stderr, "SHN_XINDEX found, but no symtab shndx section.\n");
+ return -1;
+ }
+
+ if (gelf_getsymshndx(symtab->syms, symtab->syms_shndx, core_id, &sym, &xndx))
+ sym.st_shndx = xndx;
+ }
+
if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
return -1;
if (collect_function(btfe, &sym))
diff --git a/elf_symtab.c b/elf_symtab.c
index 741990ea3ed9..c1def0189652 100644
--- a/elf_symtab.c
+++ b/elf_symtab.c
@@ -17,11 +17,13 @@
struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
{
+ size_t index;
+
if (name == NULL)
name = ".symtab";
GElf_Shdr shdr;
- Elf_Scn *sec = elf_section_by_name(elf, ehdr, &shdr, name, NULL);
+ Elf_Scn *sec = elf_section_by_name(elf, ehdr, &shdr, name, &index);
if (sec == NULL)
return NULL;
@@ -29,6 +31,8 @@ struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
if (gelf_getshdr(sec, &shdr) == NULL)
return NULL;
+ int xindex = elf_scnshndx(sec);
+
struct elf_symtab *symtab = malloc(sizeof(*symtab));
if (symtab == NULL)
return NULL;
@@ -49,6 +53,31 @@ struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
if (symtab->symstrs == NULL)
goto out_free_name;
+ /*
+ * The .symtab section has optional extended section index
+ * table, load its data so it can be used to resolve symbol's
+ * section index.
+ **/
+ if (xindex > 0) {
+ GElf_Shdr shdr_shndx;
+ Elf_Scn *sec_shndx;
+
+ sec_shndx = elf_getscn(elf, xindex);
+ if (sec_shndx == NULL)
+ goto out_free_name;
+
+ if (gelf_getshdr(sec_shndx, &shdr_shndx) == NULL)
+ goto out_free_name;
+
+ /* Extra check to verify it belongs to the .symtab */
+ if (index != shdr_shndx.sh_link)
+ goto out_free_name;
+
+ symtab->syms_shndx = elf_getdata(elf_getscn(elf, xindex), NULL);
+ if (symtab->syms_shndx == NULL)
+ goto out_free_name;
+ }
+
symtab->nr_syms = shdr.sh_size / shdr.sh_entsize;
return symtab;
diff --git a/elf_symtab.h b/elf_symtab.h
index 359add69c8ab..f9277a48ed4c 100644
--- a/elf_symtab.h
+++ b/elf_symtab.h
@@ -16,6 +16,7 @@ struct elf_symtab {
uint32_t nr_syms;
Elf_Data *syms;
Elf_Data *symstrs;
+ Elf_Data *syms_shndx;
char *name;
};
--
2.27.0
next prev parent reply other threads:[~2021-01-19 22:15 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-19 22:12 [PATCH 0/3] dwarves,libbpf: Add support to use optional extended section index table Jiri Olsa
2021-01-19 22:12 ` [PATCH 1/3] elf_symtab: Add support for SHN_XINDEX index to elf_section_by_name Jiri Olsa
2021-01-20 1:23 ` Andrii Nakryiko
2021-01-20 12:20 ` Jiri Olsa
2021-01-19 22:12 ` Jiri Olsa [this message]
2021-01-20 2:03 ` [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values Andrii Nakryiko
2021-01-20 12:25 ` Jiri Olsa
2021-01-19 22:12 ` [PATCH bpf-next 3/3] libbpf: Use string table index from index table if needed Jiri Olsa
2021-01-20 1:22 ` Andrii Nakryiko
2021-01-20 11:12 ` Arnaldo Carvalho de Melo
2021-01-20 12:26 ` Jiri Olsa
2021-01-19 23:17 ` [PATCH 0/3] dwarves,libbpf: Add support to use optional extended section index table Joe Lawrence
2021-01-21 20:22 [PATCHv2 " Jiri Olsa
2021-01-21 20:22 ` [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values Jiri Olsa
2021-01-21 23:32 ` Andrii Nakryiko
2021-01-22 9:32 ` Jiri Olsa
2021-01-22 20:46 ` Jiri Olsa
2021-01-22 22:55 ` Andrii Nakryiko
2021-01-23 18:51 ` Jiri Olsa
2021-01-23 20:07 ` Andrii Nakryiko
2021-01-23 20:21 ` Mark Wielaard
2021-01-23 20:08 ` Mark Wielaard
2021-01-23 20:15 ` Jiri Olsa
2021-01-23 21:23 ` Jiri Olsa
2021-01-24 6:08 ` Andrii Nakryiko
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