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Message-Id: <20210121202203.9346-3-jolsa@kernel.org>
Date: Thu, 21 Jan 2021 21:22:02 +0100
From: Jiri Olsa <jolsa@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andriin@...com>
Cc: dwarves@...r.kernel.org, netdev@...r.kernel.org,
bpf@...r.kernel.org, Yonghong Song <yhs@...com>,
Hao Luo <haoluo@...gle.com>, Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...omium.org>,
Joe Lawrence <joe.lawrence@...hat.com>,
Mark Wielaard <mjw@...hat.com>
Subject: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values
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.
Adding elf_symtab__for_each_symbol_index macro that returns
symbol's section index and usign it in collect_symbols function.
Signed-off-by: Jiri Olsa <jolsa@...nel.org>
---
btf_encoder.c | 36 ++++++++++++++++++++++++++++++++----
elf_symtab.c | 39 ++++++++++++++++++++++++++++++++++++++-
elf_symtab.h | 2 ++
3 files changed, 72 insertions(+), 5 deletions(-)
diff --git a/btf_encoder.c b/btf_encoder.c
index 5557c9efd365..6e6f22c438ce 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -585,12 +585,13 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
return 0;
}
-static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
+static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl,
+ Elf32_Word sym_sec_idx)
{
if (!fl->mcount_start &&
!strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
fl->mcount_start = sym->st_value;
- fl->mcount_sec_idx = sym->st_shndx;
+ fl->mcount_sec_idx = sym_sec_idx;
}
if (!fl->mcount_stop &&
@@ -598,9 +599,36 @@ static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
fl->mcount_stop = sym->st_value;
}
+static bool elf_sym__get(Elf_Data *syms, Elf_Data *syms_sec_idx_table,
+ int id, GElf_Sym *sym, Elf32_Word *sym_sec_idx)
+{
+ if (!gelf_getsym(syms, id, sym))
+ return false;
+
+ *sym_sec_idx = sym->st_shndx;
+
+ if (sym->st_shndx == SHN_XINDEX) {
+ if (!syms_sec_idx_table)
+ return false;
+ if (!gelf_getsymshndx(syms, syms_sec_idx_table,
+ id, sym, sym_sec_idx))
+ return false;
+ }
+
+ return true;
+}
+
+#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx) \
+ for (id = 0, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, \
+ id, &sym, &sym_sec_idx); \
+ id < symtab->nr_syms; \
+ id++, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, \
+ id, &sym, &sym_sec_idx))
+
static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
{
struct funcs_layout fl = { };
+ Elf32_Word sym_sec_idx;
uint32_t core_id;
GElf_Sym sym;
@@ -608,12 +636,12 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
percpu_var_cnt = 0;
/* search within symtab for percpu variables */
- elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
+ elf_symtab__for_each_symbol_index(btfe->symtab, core_id, sym, sym_sec_idx) {
if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
return -1;
if (collect_function(btfe, &sym))
return -1;
- collect_symbol(&sym, &fl);
+ collect_symbol(&sym, &fl, sym_sec_idx);
}
if (collect_percpu_vars) {
diff --git a/elf_symtab.c b/elf_symtab.c
index 741990ea3ed9..fad5e0c0ba3c 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 symtab_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, &symtab_index);
if (sec == NULL)
return NULL;
@@ -41,6 +43,12 @@ struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
if (symtab->syms == NULL)
goto out_free_name;
+ /*
+ * This returns extended section index table's
+ * section index, if it exists.
+ */
+ int symtab_xindex = elf_scnshndx(sec);
+
sec = elf_getscn(elf, shdr.sh_link);
if (sec == NULL)
goto out_free_name;
@@ -49,6 +57,35 @@ 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 (symtab_xindex > 0) {
+ GElf_Shdr shdr_xindex;
+ Elf_Scn *sec_xindex;
+
+ sec_xindex = elf_getscn(elf, symtab_xindex);
+ if (sec_xindex == NULL)
+ goto out_free_name;
+
+ if (gelf_getshdr(sec_xindex, &shdr_xindex) == NULL)
+ goto out_free_name;
+
+ /* Extra check to verify it's correct type */
+ if (shdr_xindex.sh_type != SHT_SYMTAB_SHNDX)
+ goto out_free_name;
+
+ /* Extra check to verify it belongs to the .symtab */
+ if (symtab_index != shdr_xindex.sh_link)
+ goto out_free_name;
+
+ symtab->syms_sec_idx_table = elf_getdata(elf_getscn(elf, symtab_xindex), NULL);
+ if (symtab->syms_sec_idx_table == 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..2e05ca98158b 100644
--- a/elf_symtab.h
+++ b/elf_symtab.h
@@ -16,6 +16,8 @@ struct elf_symtab {
uint32_t nr_syms;
Elf_Data *syms;
Elf_Data *symstrs;
+ /* Data of SHT_SYMTAB_SHNDX section. */
+ Elf_Data *syms_sec_idx_table;
char *name;
};
--
2.27.0
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