Re: [PATCH v5 09/13] module: Move kallsyms support into a separate file

[Aaron Tomlin]

On Thu 2022-02-10 13:43 +0000, Christophe Leroy wrote:
> Checkpatch reports:
>
> total: 3 errors, 1 warnings, 26 checks, 1103 lines checked

Christophe,

> Sparse reports the following:
>
>    CHECK   kernel/module/kallsyms.c
> kernel/module/kallsyms.c:174:23: warning: incorrect type in assignment
> (different address spaces)
> kernel/module/kallsyms.c:174:23:    expected struct mod_kallsyms
> [noderef] __rcu *kallsyms
> kernel/module/kallsyms.c:174:23:    got void *


Thanks once again for your review and feedback!

Indeed I can see the same via 'make C=2 kernel/module/'. Looking at struct
'module' declaration we see that field namely "kallsyms" has the __rcu
marker. So, If I understand correctly, perhaps this can be resolved as
follows, to be more explicit:

@@ -171,7 +171,7 @@ void add_kallsyms(struct module *mod, const struct
load_info *info)
        Elf_Shdr *symsec = &info->sechdrs[info->index.sym];

        /* Set up to point into init section. */
-       mod->kallsyms = mod->init_layout.base + info->mod_kallsyms_init_off;
+       mod->kallsyms = (struct mod_kallsyms __rcu
*)mod->init_layout.base + info->mod_kallsyms_init_off;


> kernel/module/kallsyms.c:176:12: warning: dereference of noderef expression
> kernel/module/kallsyms.c:177:12: warning: dereference of noderef expression
> kernel/module/kallsyms.c:179:12: warning: dereference of noderef expression
> kernel/module/kallsyms.c:180:12: warning: dereference of noderef expression
> kernel/module/kallsyms.c:189:18: warning: dereference of noderef expression
> kernel/module/kallsyms.c:190:35: warning: dereference of noderef expression
> kernel/module/kallsyms.c:191:20: warning: dereference of noderef expression
> kernel/module/kallsyms.c:196:32: warning: dereference of noderef expression
> kernel/module/kallsyms.c:199:45: warning: dereference of noderef expression

I will use rcu_dereference*() for the above since the pointer should not be
accessed directly.

> >
> > diff --git a/kernel/module/Makefile b/kernel/module/Makefile
> > index 62c9fc91d411..868b13c06920 100644
> > --- a/kernel/module/Makefile
> > +++ b/kernel/module/Makefile
> > @@ -12,4 +12,5 @@ obj-$(CONFIG_LIVEPATCH) += livepatch.o
> >   obj-$(CONFIG_MODULES_TREE_LOOKUP) += tree_lookup.o
> >   obj-$(CONFIG_STRICT_MODULE_RWX) += strict_rwx.o
> >   obj-$(CONFIG_DEBUG_KMEMLEAK) += debug_kmemleak.o
> > +obj-$(CONFIG_KALLSYMS) += kallsyms.o
> >   endif
> > diff --git a/kernel/module/internal.h b/kernel/module/internal.h
> > index 33d7befd0602..7973666452c3 100644
> > --- a/kernel/module/internal.h
> > +++ b/kernel/module/internal.h
> > @@ -69,6 +69,11 @@ struct load_info {
> >   };
> >
> >   int mod_verify_sig(const void *mod, struct load_info *info);
> > +struct module *find_module_all(const char *name, size_t len, bool even_unformed);
> > +unsigned long kernel_symbol_value(const struct kernel_symbol *sym);
>
> This function is small enought to be a 'static inline' in internal.h

Fair enough.

> > +int cmp_name(const void *name, const void *sym);
> > +long get_offset(struct module *mod, unsigned int *size, Elf_Shdr *sechdr,
> > +               unsigned int section);
>
> Having a non static function called get_offset() seems dangerous.
>
> There are already several get_offset() functions in the kernel allthough
> they are all static.
>
> It takes a struct module as an argument so it could be called
> module_get_offset()

The rename is a good idea.

> > +bool sect_empty(const Elf_Shdr *sect);
>
> sect_empty() is small enough to remain a static inline.

Yes and moved to kernel/module/internal.h.

>
> > +const char *find_kallsyms_symbol(struct module *mod, unsigned long addr,
> > +                    unsigned long *size, unsigned long *offset);
>
> This is not used outside kallsyms.c, no need to have it in internal.h

Agreed.

>
> > +#else /* !CONFIG_KALLSYMS */
> > +static inline void layout_symtab(struct module *mod, struct load_info *info) { }
> > +static inline void add_kallsyms(struct module *mod, const struct load_info *info) { }
> > +static inline char *find_kallsyms_symbol(struct module *mod, unsigned long addr,
> > +                     unsigned long *size, unsigned long *offset)
>
> This is not used outside kallsyms.c, no need to have it when
> !CONFIG_KALLSYMS

Agreed.

>
> > +{
> > +    return NULL;
> > +}
> > +#endif /* CONFIG_KALLSYMS */
> > diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> > new file mode 100644
> > index 000000000000..ed28f6310701
> > --- /dev/null
> > +++ b/kernel/module/kallsyms.c
> > @@ -0,0 +1,502 @@
> ...
> > +
> > +/* Given a module and name of symbol, find and return the symbol's value */
> > +static unsigned long find_kallsyms_symbol_value(struct module *mod, const char *name)
>
> This function is called from main.c, it can't be static and must be
> defined in internal.h

Agreed. This was an unfortunate oversight.

> > -static unsigned long kernel_symbol_value(const struct kernel_symbol *sym)
> > +unsigned long kernel_symbol_value(const struct kernel_symbol *sym)
>
> This function is small enought to become a 'static inline' in internal.h

Agreed.

> > +int cmp_name(const void *name, const void *sym)
>
> This function is small enought to become a 'static inline' in internal.h

Agreed.


Kind regards,

-- 
Aaron Tomlin