| Message ID | 20240815173903.4172139-25-samitolvanen@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Implement DWARF modversions | expand |
On Fri, Aug 16, 2024 at 2:39 AM Sami Tolvanen <samitolvanen@google.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@google.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 >
On Wed, Aug 28, 2024 at 3:50 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > On Fri, Aug 16, 2024 at 2:39 AM Sami Tolvanen <samitolvanen@google.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@google.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? > Presumably, this is an unfortunate duplication, but I do not have an idea to avoid it. The symbol reference in assembly code works in *.S as well as *.c. The C reference will pull-in the debug info, but it will not work in *.S -- Best Regards Masahiro Yamada
On Wed, Aug 28, 2024 at 04:15:03PM +0900, Masahiro Yamada wrote: > On Wed, Aug 28, 2024 at 3:50 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > > > On Fri, Aug 16, 2024 at 2:39 AM Sami Tolvanen <samitolvanen@google.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@google.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? > > > > > Presumably, this is an unfortunate duplication, but I do not have an > idea to avoid it. > > The symbol reference in assembly code works in *.S as well as *.c. > > The C reference will pull-in the debug info, but it will not work in *.S Correct. I'm not a huge fan of the extra reference either, but I don't see a cleaner way to ensure we always have all the types in DWARF. Sami
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)) {
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@google.com> --- scripts/gendwarfksyms/dwarf.c | 26 +++++++++++++++++++++++++- scripts/gendwarfksyms/gendwarfksyms.h | 6 ++++++ scripts/gendwarfksyms/symbols.c | 16 ++++++++++++++++ 3 files changed, 47 insertions(+), 1 deletion(-)