Re: [PATCH] kallsyms: store type information in its own array

[Jessica Yu]

+++ Eugene Loh [07/03/19 15:37 -0800]:
> It's been 1.5 weeks.  Could I get some feedback on this patch? Thanks.

Apologies, I was on vacation the past week.

Added some CC's from the last time we talked about overwriting the
st_size/st_info Elf_Sym fields, in case anyone else has comments.

> On 02/25/2019 11:59 AM, eugene.loh@xxxxxxxxxx wrote:
> > From: Eugene Loh <eugene.loh@xxxxxxxxxx>
> >
> > When a module is loaded, its symbols' Elf_Sym information is stored
> > in a symtab.  Further, type information is also captured.  Since
> > Elf_Sym has no type field, historically the st_info field has been
> > hijacked for storing type:  st_info was overwritten.
> >
> > commit 5439c985c5a83a8419f762115afdf560ab72a452 ("module: Overwrite
> > st_size instead of st_info") changes that practice, as its one-liner
> > indicates.  Unfortunately, this change overwrites symbol size,
> > information that a tool like DTrace expects to find.
> >
> > Allocate a typetab array to store type information so that no Elf_Sym
> > field needs to be overwritten.
> >
> > Fixes: 5439c985c5a8 ("module: Overwrite st_size instead of st_info")
> > Signed-off-by: Eugene Loh <eugene.loh@xxxxxxxxxx>
> > Reviewed-by: Nick Alcock <nick.alcock@xxxxxxxxxx>
> > ---
> >  include/linux/module.h   |  1 +
> >  kernel/module-internal.h |  2 +-
> >  kernel/module.c          | 21 ++++++++++++++-------
> >  3 files changed, 16 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/module.h b/include/linux/module.h
> > index f5bc4c0..9c1bc21 100644
> > --- a/include/linux/module.h
> > +++ b/include/linux/module.h
> > @@ -315,6 +315,7 @@ struct mod_kallsyms {
> >  	Elf_Sym *symtab;
> >  	unsigned int num_symtab;
> >  	char *strtab;
> > +	char *typetab;
> >  };
> >  #ifdef CONFIG_LIVEPATCH
> > diff --git a/kernel/module-internal.h b/kernel/module-internal.h
> > index 79c9be2..67828af 100644
> > --- a/kernel/module-internal.h
> > +++ b/kernel/module-internal.h
> > @@ -20,7 +20,7 @@ struct load_info {
> >  	unsigned long len;
> >  	Elf_Shdr *sechdrs;
> >  	char *secstrings, *strtab;
> > -	unsigned long symoffs, stroffs;
> > +	unsigned long symoffs, stroffs, init_typeoff, core_typeoff;

Small nit: typeoff to typeoffs to match symoffs and stroffs.

Otherwise the rest looks fine to me. I guess it's about time we get
rid of the Elf_Sym field hijacking anyway :-)

Thanks!

Jessica

> >  	struct _ddebug *debug;
> >  	unsigned int num_debug;
> >  	bool sig_ok;
> > diff --git a/kernel/module.c b/kernel/module.c
> > index 2ad1b52..c7c1bcd 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -2647,6 +2647,8 @@ static void layout_symtab(struct module *mod, struct load_info *info)
> >  	info->symoffs = ALIGN(mod->core_layout.size, symsect->sh_addralign ?: 1);
> >  	info->stroffs = mod->core_layout.size = info->symoffs + ndst * sizeof(Elf_Sym);
> >  	mod->core_layout.size += strtab_size;
> > +	info->core_typeoff = mod->core_layout.size;
> > +	mod->core_layout.size += ndst * sizeof(char);
> >  	mod->core_layout.size = debug_align(mod->core_layout.size);
> >  	/* Put string table section at end of init part of module. */
> > @@ -2660,6 +2662,8 @@ static void layout_symtab(struct module *mod, struct load_info *info)
> >  				      __alignof__(struct mod_kallsyms));
> >  	info->mod_kallsyms_init_off = mod->init_layout.size;
> >  	mod->init_layout.size += sizeof(struct mod_kallsyms);
> > +	info->init_typeoff = mod->init_layout.size;
> > +	mod->init_layout.size += nsrc * sizeof(char);
> >  	mod->init_layout.size = debug_align(mod->init_layout.size);
> >  }
> > @@ -2683,20 +2687,23 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
> >  	mod->kallsyms->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
> >  	/* Make sure we get permanent strtab: don't use info->strtab. */
> >  	mod->kallsyms->strtab = (void *)info->sechdrs[info->index.str].sh_addr;
> > +	mod->kallsyms->typetab = mod->init_layout.base + info->init_typeoff;
> > -	/* Set types up while we still have access to sections. */
> > -	for (i = 0; i < mod->kallsyms->num_symtab; i++)
> > -		mod->kallsyms->symtab[i].st_size
> > -			= elf_type(&mod->kallsyms->symtab[i], info);
> > -
> > -	/* Now populate the cut down core kallsyms for after init. */
> > +	/*
> > +	 * Now populate the cut down core kallsyms for after init
> > +	 * and set types up while we still have access to sections.
> > +	 */
> >  	mod->core_kallsyms.symtab = dst = mod->core_layout.base + info->symoffs;
> >  	mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs;
> > +	mod->core_kallsyms.typetab = mod->core_layout.base + info->core_typeoff;
> >  	src = mod->kallsyms->symtab;
> >  	for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) {
> > +		mod->kallsyms->typetab[i] = elf_type(src + i, info);
> >  		if (i == 0 || is_livepatch_module(mod) ||
> >  		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
> >  				   info->index.pcpu)) {
> > +			mod->core_kallsyms.typetab[ndst] =
> > +			    mod->kallsyms->typetab[i];
> >  			dst[ndst] = src[i];
> >  			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
> >  			s += strlcpy(s, &mod->kallsyms->strtab[src[i].st_name],
> > @@ -4084,7 +4091,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
> >  			const Elf_Sym *sym = &kallsyms->symtab[symnum];
> >  			*value = kallsyms_symbol_value(sym);
> > -			*type = sym->st_size;
> > +			*type = kallsyms->typetab[symnum];
> >  			strlcpy(name, kallsyms_symbol_name(kallsyms, symnum), KSYM_NAME_LEN);
> >  			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
> >  			*exported = is_exported(name, *value, mod);