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Message-ID: <CAEf4BzbC-s=27vmcJ1KYLVKgGbns2py1bHny3Q_yr4v3Oe49RQ@mail.gmail.com>
Date: Fri, 22 Jan 2021 12:16:42 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andriin@...com>, dwarves@...r.kernel.org,
Networking <netdev@...r.kernel.org>, bpf <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: Re: [PATCH 2/2] bpf_encoder: Translate SHN_XINDEX in symbol's
st_shndx values
On Fri, Jan 22, 2021 at 8:46 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> 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@...nel.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
> {
> 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
> {
> 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?
> +{
> + 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?
> + }
> +
> + 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...
And please do fix elf_symtab__for_each_symbol().
> +
> 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;
>
[...]
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