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Message-ID: <CAK7LNATb8zbwEVgM+_=CaYSysov6YBMbKjy8+t8CrRA_72jxuQ@mail.gmail.com>
Date: Wed, 28 Aug 2024 15:50:03 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Luis Chamberlain <mcgrof@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Matthew Maurer <mmaurer@...gle.com>,
Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>,
Petr Pavlu <petr.pavlu@...e.com>, Neal Gompa <neal@...pa.dev>, Hector Martin <marcan@...can.st>,
Janne Grunau <j@...nau.net>, Asahi Linux <asahi@...ts.linux.dev>, linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org,
rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 04/19] gendwarfksyms: Add support for type pointers
On Fri, Aug 16, 2024 at 2:39 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> The compiler may choose not to emit type information in DWARF for
> external symbols. Clang, for example, does this for symbols not
> defined in the current TU.
>
> To provide a way to work around this issue, add support for
> __gendwarfksyms_ptr_<symbol> pointers that force the compiler to emit
> the necessary type information in DWARF also for the missing symbols.
>
> Example usage:
>
> #define GENDWARFKSYMS_PTR(sym) \
> static typeof(sym) *__gendwarfksyms_ptr_##sym __used \
> __section(".discard.gendwarfksyms") = &sym;
>
> extern int external_symbol(void);
> GENDWARFKSYMS_PTR(external_symbol);
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
Commit ddb5cdbafaaad6b99d7007ae1740403124502d03
had a similar idea; it has a reference to each
export symbol, including the ones defined in different TUs,
but in assembly code.
Didn't it suffice your need?
> ---
> scripts/gendwarfksyms/dwarf.c | 26 +++++++++++++++++++++++++-
> scripts/gendwarfksyms/gendwarfksyms.h | 6 ++++++
> scripts/gendwarfksyms/symbols.c | 16 ++++++++++++++++
> 3 files changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/gendwarfksyms/dwarf.c b/scripts/gendwarfksyms/dwarf.c
> index 71cfab0553da..956b30224316 100644
> --- a/scripts/gendwarfksyms/dwarf.c
> +++ b/scripts/gendwarfksyms/dwarf.c
> @@ -94,6 +94,28 @@ static int process_variable(struct state *state, Dwarf_Die *die)
> return check(process(state, "variable;\n"));
> }
>
> +static int process_symbol_ptr(struct state *state, Dwarf_Die *die)
> +{
> + Dwarf_Die ptr_type;
> + Dwarf_Die type;
> +
> + if (!get_ref_die_attr(die, DW_AT_type, &ptr_type) ||
> + dwarf_tag(&ptr_type) != DW_TAG_pointer_type) {
> + error("%s must be a pointer type!", get_name(die));
> + return -1;
> + }
> +
> + if (!get_ref_die_attr(&ptr_type, DW_AT_type, &type)) {
> + error("%s pointer missing a type attribute?", get_name(die));
> + return -1;
> + }
> +
> + if (dwarf_tag(&type) == DW_TAG_subroutine_type)
> + return check(process_subprogram(state, &type));
> + else
> + return check(process_variable(state, &ptr_type));
> +}
> +
> static int process_exported_symbols(struct state *state, Dwarf_Die *die)
> {
> int tag = dwarf_tag(die);
> @@ -114,7 +136,9 @@ static int process_exported_symbols(struct state *state, Dwarf_Die *die)
>
> debug("%s", state->sym->name);
>
> - if (tag == DW_TAG_subprogram)
> + if (is_symbol_ptr(get_name(&state->die)))
> + check(process_symbol_ptr(state, &state->die));
> + else if (tag == DW_TAG_subprogram)
> check(process_subprogram(state, &state->die));
> else
> check(process_variable(state, &state->die));
> diff --git a/scripts/gendwarfksyms/gendwarfksyms.h b/scripts/gendwarfksyms/gendwarfksyms.h
> index cb9106dfddb9..8f6acd1b8f8f 100644
> --- a/scripts/gendwarfksyms/gendwarfksyms.h
> +++ b/scripts/gendwarfksyms/gendwarfksyms.h
> @@ -61,6 +61,11 @@ extern bool debug;
> /*
> * symbols.c
> */
> +
> +/* See symbols.c:is_symbol_ptr */
> +#define SYMBOL_PTR_PREFIX "__gendwarfksyms_ptr_"
> +#define SYMBOL_PTR_PREFIX_LEN (sizeof(SYMBOL_PTR_PREFIX) - 1)
> +
> struct symbol_addr {
> uint32_t section;
> Elf64_Addr address;
> @@ -78,6 +83,7 @@ struct symbol {
> struct hlist_node name_hash;
> };
>
> +extern bool is_symbol_ptr(const char *name);
> extern int symbol_read_exports(FILE *file);
> extern int symbol_read_symtab(int fd);
> extern struct symbol *symbol_get(const char *name);
> diff --git a/scripts/gendwarfksyms/symbols.c b/scripts/gendwarfksyms/symbols.c
> index f96acb941196..d6d016458ae1 100644
> --- a/scripts/gendwarfksyms/symbols.c
> +++ b/scripts/gendwarfksyms/symbols.c
> @@ -41,6 +41,20 @@ static int __for_each_addr(struct symbol *sym, symbol_callback_t func,
> return processed;
> }
>
> +/*
> + * For symbols without debugging information (e.g. symbols defined in other
> + * TUs), we also match __gendwarfksyms_ptr_<symbol_name> symbols, which the
> + * kernel uses to ensure type information is present in the TU that exports
> + * the symbol. A __gendwarfksyms_ptr pointer must have the same type as the
> + * exported symbol, e.g.:
> + *
> + * typeof(symname) *__gendwarf_ptr_symname = &symname;
> + */
> +bool is_symbol_ptr(const char *name)
> +{
> + return name && !strncmp(name, SYMBOL_PTR_PREFIX, SYMBOL_PTR_PREFIX_LEN);
> +}
> +
> static int for_each(const char *name, bool name_only, symbol_callback_t func,
> void *data)
> {
> @@ -49,6 +63,8 @@ static int for_each(const char *name, bool name_only, symbol_callback_t func,
>
> if (!name || !*name)
> return 0;
> + if (is_symbol_ptr(name))
> + name += SYMBOL_PTR_PREFIX_LEN;
>
> hash_for_each_possible_safe(symbol_names, match, tmp, name_hash,
> name_hash(name)) {
> --
> 2.46.0.184.g6999bdac58-goog
>
--
Best Regards
Masahiro Yamada
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