On Fri, Jan 22, 2021 at 12:16:42PM -0800, Andrii Nakryiko wrote:

On Fri, Jan 22, 2021 at 8:46 AM Jiri Olsa [off-list ref] wrote:

quoted

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 functions.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 btf_encoder.c | 59 +++++++++++++++++++++++++++++++++++++--------------
 elf_symtab.c  | 39 +++++++++++++++++++++++++++++++++-
 elf_symtab.h  |  2 ++
 3 files changed, 83 insertions(+), 17 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 5557c9efd365..56ee55965093 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c

@@ -63,13 +63,13 @@ static void delete_functions(void)
 #define max(x, y) ((x) < (y) ? (y) : (x))
 #endif

-static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
+static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
+                           Elf32_Word sym_sec_idx)

nit: we use size_t or int for this, no need for libelf types here, imo

ok

quoted

 {
        struct elf_function *new;
        static GElf_Shdr sh;
-       static int last_idx;
+       static Elf32_Word last_idx;
        const char *name;
-       int idx;

        if (elf_sym__type(sym) != STT_FUNC)
                return 0;

@@ -90,12 +90,10 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
                functions = new;
        }

-       idx = elf_sym__section(sym);
-
-       if (idx != last_idx) {
-               if (!elf_section_by_idx(btfe->elf, &sh, idx))
+       if (sym_sec_idx != last_idx) {
+               if (!elf_section_by_idx(btfe->elf, &sh, sym_sec_idx))
                        return 0;
-               last_idx = idx;
+               last_idx = sym_sec_idx;
        }

        functions[functions_cnt].name = name;

@@ -542,14 +540,15 @@ static bool percpu_var_exists(uint64_t addr, uint32_t *sz, const char **name)
        return true;
 }

-static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
+static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym,
+                             Elf32_Word sym_sec_idx)

nit: same, size_t or just int would be fine

quoted

 {
        const char *sym_name;
        uint64_t addr;
        uint32_t size;

        /* compare a symbol's shndx to determine if it's a percpu variable */
-       if (elf_sym__section(sym) != btfe->percpu_shndx)
+       if (sym_sec_idx != btfe->percpu_shndx)
                return 0;
        if (elf_sym__type(sym) != STT_OBJECT)
                return 0;

@@ -585,12 +584,13 @@ 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 +598,36 @@ 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)

This is a generic function, why don't you want to move it into elf_symtab.h?

quoted

+{
+       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;

You also ignored my feedback about not fetching symbol twice. Why?

ugh, I did not read your last 2 comments.. let me check that, sry

quoted

+       }
+
+       return true;
+}
+
+#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx)                \
+       for (id = 0;                                                            \
+            id < symtab->nr_syms &&                                            \
+            elf_sym__get(symtab->syms, symtab->syms_sec_idx_table,             \
+                         id, &sym, &sym_sec_idx);                              \
+            id++)

This should be in elf_symtab.h next to elf_symtab__for_each_symbol.

And thinking a bit more, the variant with just ignoring symbols that
we failed to get is probably a safer alternative. I.e., currently
there is no way to communicate that we terminated iteration with
error, so it's probably better to skip failed symbols and still get
the rest, no? I was hoping to discuss stuff like this on the previous
version of the patch...

I was thinking of that, but then I thought it's better to fail,
than have possibly wrong data in BTF, because the ELF data is
possibly damaged? no idea.. so I took the safer way

jirka