Re: [PATCH v3 04/20] gendwarfksyms: Add address matching

[Petr Pavlu <petr.pavlu@suse.com>]

On 9/23/24 20:18, Sami Tolvanen wrote:
> The compiler may choose not to emit type information in DWARF for all
> aliases, but it's possible for each alias to be exported separately.
> To ensure we find type information for the aliases as well, read
> {section, address} tuples from the symbol table and match symbols also
> by address.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>  scripts/gendwarfksyms/gendwarfksyms.c |   2 +
>  scripts/gendwarfksyms/gendwarfksyms.h |  13 +++
>  scripts/gendwarfksyms/symbols.c       | 153 +++++++++++++++++++++++++-
>  3 files changed, 165 insertions(+), 3 deletions(-)
> 
> diff --git a/scripts/gendwarfksyms/gendwarfksyms.c b/scripts/gendwarfksyms/gendwarfksyms.c
> index 096a334fa5b3..5032ec487626 100644
> --- a/scripts/gendwarfksyms/gendwarfksyms.c
> +++ b/scripts/gendwarfksyms/gendwarfksyms.c
> @@ -105,6 +105,8 @@ int main(int argc, char **argv)
>  			return -1;
>  		}
>  
> +		symbol_read_symtab(fd);
> +
>  		dwfl = dwfl_begin(&callbacks);
>  		if (!dwfl) {
>  			error("dwfl_begin failed for '%s': %s", argv[n],
> diff --git a/scripts/gendwarfksyms/gendwarfksyms.h b/scripts/gendwarfksyms/gendwarfksyms.h
> index 1a10d18f178e..a058647e2361 100644
> --- a/scripts/gendwarfksyms/gendwarfksyms.h
> +++ b/scripts/gendwarfksyms/gendwarfksyms.h
> @@ -66,14 +66,27 @@ extern int dump_dies;
>   * symbols.c
>   */
>  
> +static inline unsigned int addr_hash(uintptr_t addr)
> +{
> +	return hash_ptr((const void *)addr);
> +}
> +
> +struct symbol_addr {
> +	uint32_t section;
> +	Elf64_Addr address;
> +};
> +
>  struct symbol {
>  	const char *name;
> +	struct symbol_addr addr;
> +	struct hlist_node addr_hash;
>  	struct hlist_node name_hash;
>  };
>  
>  typedef void (*symbol_callback_t)(struct symbol *, void *arg);
>  
>  void symbol_read_exports(FILE *file);
> +void symbol_read_symtab(int fd);
>  struct symbol *symbol_get(const char *name);
>  
>  /*
> diff --git a/scripts/gendwarfksyms/symbols.c b/scripts/gendwarfksyms/symbols.c
> index 1809be93d18c..d84b46675dd1 100644
> --- a/scripts/gendwarfksyms/symbols.c
> +++ b/scripts/gendwarfksyms/symbols.c
> @@ -6,9 +6,41 @@
>  #include "gendwarfksyms.h"
>  
>  #define SYMBOL_HASH_BITS 15
> +
> +/* struct symbol_addr -> struct symbol */
> +static HASHTABLE_DEFINE(symbol_addrs, 1 << SYMBOL_HASH_BITS);
> +/* name -> struct symbol */
>  static HASHTABLE_DEFINE(symbol_names, 1 << SYMBOL_HASH_BITS);
>  
> -static int for_each(const char *name, symbol_callback_t func, void *data)
> +static inline unsigned int symbol_addr_hash(const struct symbol_addr *addr)
> +{
> +	return hash_32(addr->section ^ addr_hash(addr->address));
> +}
> +
> +static int __for_each_addr(struct symbol *sym, symbol_callback_t func,
> +			   void *data)
> +{
> +	struct hlist_node *tmp;
> +	struct symbol *match = NULL;
> +	int processed = 0;
> +
> +	hash_for_each_possible_safe(symbol_addrs, match, tmp, addr_hash,
> +				    symbol_addr_hash(&sym->addr)) {
> +		if (match == sym)
> +			continue; /* Already processed */
> +
> +		if (match->addr.section == sym->addr.section &&
> +		    match->addr.address == sym->addr.address) {
> +			func(match, data);
> +			++processed;
> +		}
> +	}
> +
> +	return processed;
> +}
> +
> +static int for_each(const char *name, bool name_only, symbol_callback_t func,
> +		    void *data)
>  {
>  	struct hlist_node *tmp;
>  	struct symbol *match;
> @@ -21,9 +53,13 @@ static int for_each(const char *name, symbol_callback_t func, void *data)
>  		if (strcmp(match->name, name))
>  			continue;
>  
> +		/* Call func for the match, and all address matches */
>  		if (func)
>  			func(match, data);
>  
> +		if (!name_only && match->addr.section != SHN_UNDEF)
> +			return checkp(__for_each_addr(match, func, data)) + 1;
> +
>  		return 1;
>  	}
>  
> @@ -32,7 +68,7 @@ static int for_each(const char *name, symbol_callback_t func, void *data)
>  
>  static bool is_exported(const char *name)
>  {
> -	return checkp(for_each(name, NULL, NULL)) > 0;
> +	return checkp(for_each(name, true, NULL, NULL)) > 0;
>  }
>  
>  void symbol_read_exports(FILE *file)
> @@ -55,6 +91,7 @@ void symbol_read_exports(FILE *file)
>  
>  		sym = xcalloc(1, sizeof(struct symbol));
>  		sym->name = name;
> +		sym->addr.section = SHN_UNDEF;
>  
>  		hash_add(symbol_names, &sym->name_hash, hash_str(sym->name));
>  		++nsym;
> @@ -77,6 +114,116 @@ struct symbol *symbol_get(const char *name)
>  {
>  	struct symbol *sym = NULL;
>  
> -	for_each(name, get_symbol, &sym);
> +	for_each(name, false, get_symbol, &sym);
>  	return sym;
>  }

What is the reason that the for_each() call in symbol_get() is invoked
with name_only=false?

> +
> +typedef void (*elf_symbol_callback_t)(const char *name, GElf_Sym *sym,
> +				      Elf32_Word xndx, void *arg);
> +
> +static void elf_for_each_global(int fd, elf_symbol_callback_t func, void *arg)
> +{
> +	size_t sym_size;
> +	GElf_Shdr shdr_mem;
> +	GElf_Shdr *shdr;
> +	Elf_Data *xndx_data = NULL;
> +	Elf_Scn *scn;
> +	Elf *elf;
> +
> +	if (elf_version(EV_CURRENT) != EV_CURRENT)
> +		error("elf_version failed: %s", elf_errmsg(-1));
> +
> +	elf = elf_begin(fd, ELF_C_READ_MMAP, NULL);
> +	if (!elf)
> +		error("elf_begin failed: %s", elf_errmsg(-1));
> +
> +	scn = elf_nextscn(elf, NULL);
> +
> +	while (scn) {
> +		shdr = gelf_getshdr(scn, &shdr_mem);
> +
> +		if (shdr && shdr->sh_type == SHT_SYMTAB_SHNDX) {
> +			xndx_data = elf_getdata(scn, NULL);
> +			break;
> +		}
> +
> +		scn = elf_nextscn(elf, scn);
> +	}
> +
> +	sym_size = gelf_fsize(elf, ELF_T_SYM, 1, EV_CURRENT);
> +	scn = elf_nextscn(elf, NULL);
> +
> +	while (scn) {
> +		shdr = gelf_getshdr(scn, &shdr_mem);
> +
> +		if (shdr && shdr->sh_type == SHT_SYMTAB) {
> +			Elf_Data *data = elf_getdata(scn, NULL);
> +			unsigned int nsyms;
> +			unsigned int n;
> +
> +			if (shdr->sh_entsize != sym_size)
> +				error("expected sh_entsize (%lu) to be %zu",
> +				      shdr->sh_entsize, sym_size);
> +
> +			nsyms = shdr->sh_size / shdr->sh_entsize;
> +
> +			for (n = 1; n < nsyms; ++n) {
> +				const char *name = NULL;
> +				Elf32_Word xndx = 0;
> +				GElf_Sym sym_mem;
> +				GElf_Sym *sym;
> +
> +				sym = gelf_getsymshndx(data, xndx_data, n,
> +						       &sym_mem, &xndx);

Please check for sym==NULL in case the file is malformed, e.g.
.symtab_shndx is truncated.

> +
> +				if (GELF_ST_BIND(sym->st_info) == STB_LOCAL)
> +					continue;
> +
> +				if (sym->st_shndx != SHN_XINDEX)
> +					xndx = sym->st_shndx;
> +
> +				name = elf_strptr(elf, shdr->sh_link,
> +						  sym->st_name);
> +
> +				/* Skip empty symbol names */
> +				if (name && *name)
> +					func(name, sym, xndx, arg);
> +			}
> +		}
> +
> +		scn = elf_nextscn(elf, scn);
> +	}
> +
> +	check(elf_end(elf));
> +}
> +
> +static void set_symbol_addr(struct symbol *sym, void *arg)
> +{
> +	struct symbol_addr *addr = arg;
> +
> +	if (sym->addr.section == SHN_UNDEF) {
> +		sym->addr = *addr;
> +		hash_add(symbol_addrs, &sym->addr_hash,
> +			 symbol_addr_hash(&sym->addr));
> +
> +		debug("%s -> { %u, %lx }", sym->name, sym->addr.section,
> +		      sym->addr.address);
> +	} else {
> +		warn("multiple addresses for symbol %s?", sym->name);
> +	}
> +}
> +
> +static void elf_set_symbol_addr(const char *name, GElf_Sym *sym,
> +				Elf32_Word xndx, void *arg)
> +{
> +	struct symbol_addr addr = { .section = xndx, .address = sym->st_value };
> +
> +	/* Set addresses for exported symbols */
> +	if (addr.section != SHN_UNDEF)
> +		for_each(name, true, set_symbol_addr, &addr);
> +}
> +
> +void symbol_read_symtab(int fd)
> +{
> +	elf_for_each_global(fd, elf_set_symbol_addr, NULL);
> +}

-- 
Thanks,
Petr