Re: [PATCH v2 2/6] modpost: fix broken sym->namespace for external module builds

[Masahiro Yamada <yamada.masahiro@socionext.com>]

Hi Shaun,

On Thu, Oct 3, 2019 at 10:29 PM Shaun Ruffell <sruffell@sruffell.net> wrote:
>
> On Thu, Oct 03, 2019 at 04:58:22PM +0900, Masahiro Yamada wrote:
> > Currently, external module builds produce tons of false-positives:
> >
> >   WARNING: module <mod> uses symbol <sym> from namespace <ns>, but does not import it.
> >
> > Here, the <ns> part shows a random string.
> >
> > When you build external modules, the symbol info of vmlinux and
> > in-kernel modules are read from $(objtree)/Module.symvers, but
> > read_dump() is buggy in multiple ways:
> >
> > [1] When the modpost is run for vmlinux and in-kernel modules,
> > sym_extract_namespace() allocates memory for the namespace. On the
> > other hand, read_dump() does not, then sym->namespace will point to
> > somewhere in the line buffer of get_next_line(). The data in the
> > buffer will be replaced soon, and sym->namespace will end up with
> > pointing to unrelated data. As a result, check_exports() will show
> > random strings in the warning messages.
> >
> > [2] When there is no namespace, sym_extract_namespace() returns NULL.
> > On the other hand, read_dump() sets namespace to an empty string "".
> > (but, it will be later replaced with unrelated data due to bug [1].)
> > The check_exports() shows a warning unless exp->namespace is NULL,
> > so every symbol read from read_dump() emits the warning, which is
> > mostly false positive.
> >
> > To address [1], sym_add_exported() calls strdup() for s->namespace.
> > The namespace from sym_extract_namespace() must be freed to avoid
> > memory leak.
> >
> > For [2], I changed the if-conditional in check_exports().
> >
> > This commit also fixes sym_add_exported() to set s->namespace correctly
> > when the symbol is preloaded.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Reviewed-by: Matthias Maennich <maennich@google.com>
> > ---
> >
> > Changes in v2:
> >   - Change the approach to deal with ->preloaded
> >
> >  scripts/mod/modpost.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > index 2c644086c412..936d3ad23c83 100644
> > --- a/scripts/mod/modpost.c
> > +++ b/scripts/mod/modpost.c
> > @@ -166,7 +166,7 @@ struct symbol {
> >       struct module *module;
> >       unsigned int crc;
> >       int crc_valid;
> > -     const char *namespace;
> > +     char *namespace;
> >       unsigned int weak:1;
> >       unsigned int vmlinux:1;    /* 1 if symbol is defined in vmlinux */
> >       unsigned int kernel:1;     /* 1 if symbol is from kernel
> > @@ -348,7 +348,7 @@ static enum export export_from_sec(struct elf_info *elf, unsigned int sec)
> >               return export_unknown;
> >  }
> >
> > -static const char *sym_extract_namespace(const char **symname)
> > +static char *sym_extract_namespace(const char **symname)
> >  {
> >       char *namespace = NULL;
> >       char *ns_separator;
> > @@ -373,7 +373,6 @@ static struct symbol *sym_add_exported(const char *name, const char *namespace,
> >
> >       if (!s) {
> >               s = new_symbol(name, mod, export);
> > -             s->namespace = namespace;
> >       } else {
> >               if (!s->preloaded) {
> >                       warn("%s: '%s' exported twice. Previous export was in %s%s\n",
> > @@ -384,6 +383,8 @@ static struct symbol *sym_add_exported(const char *name, const char *namespace,
> >                       s->module = mod;
> >               }
> >       }
> > +     free(s->namespace);
> > +     s->namespace = namespace ? strdup(namespace) : NULL;
> >       s->preloaded = 0;
> >       s->vmlinux   = is_vmlinux(mod->name);
> >       s->kernel    = 0;
> > @@ -670,7 +671,8 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
> >       unsigned int crc;
> >       enum export export;
> >       bool is_crc = false;
> > -     const char *name, *namespace;
> > +     const char *name;
> > +     char *namespace;
> >
> >       if ((!is_vmlinux(mod->name) || mod->is_dot_o) &&
> >           strstarts(symname, "__ksymtab"))
> > @@ -745,6 +747,7 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
> >                       name = symname + strlen("__ksymtab_");
> >                       namespace = sym_extract_namespace(&name);
> >                       sym_add_exported(name, namespace, mod, export);
> > +                     free(namespace);
> >               }
> >               if (strcmp(symname, "init_module") == 0)
> >                       mod->has_init = 1;
> > @@ -2193,7 +2196,7 @@ static int check_exports(struct module *mod)
> >               else
> >                       basename = mod->name;
> >
> > -             if (exp->namespace) {
> > +             if (exp->namespace && exp->namespace[0]) {
> >                       add_namespace(&mod->required_namespaces,
> >                                     exp->namespace);
> >
>
> This looks good to me and is better than what I had originally proposed.
> I confirmed that I can still build an external module without any
> warnings. (But I did have to convince myself that it was OK to store
> empty namespace strings in the symbol structure and that check_exports()
> would cover it sufficiently)

You have a point.

The change to check_exports() looks strange.
It is actually related to my previous patch submission.

See this patch:
https://lore.kernel.org/patchwork/patch/1131970/


Currently, the NULL pointer means no namespace.

I noticed passing an empty string as the namespace
simplified <linux/export.h> a lot.

So, I changed it in a way that
an empty string also means no namespace.


Anyway, Matthias took over the refactoring work.
So, I am not sure if this change is still helpful...


If it is no longer useful, I am happy to send v3
with the following:


diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 442d5e2ad688..bdd3956e89c9 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -2453,6 +2453,8 @@ static void read_dump(const char *fname,
unsigned int kernel)
                        mod = new_module(modname);
                        mod->skip = 1;
                }
+               if (namespace[0] == '\0')
+                       namespace = NULL;
                s = sym_add_exported(symname, namespace, mod,
                                     export_no(export));
                s->kernel    = kernel;




-- 
Best Regards
Masahiro Yamada