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

From: Jessica Yu
Date: Fri Oct 04 2019 - 11:10:10 EST

+++ Masahiro Yamada [03/10/19 23:42 +0900]:
Hi Shaun,

On Thu, Oct 3, 2019 at 10:29 PM Shaun Ruffell <sruffell@xxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Reviewed-by: Matthias Maennich <maennich@xxxxxxxxxx>
> ---
>
> 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...

Hm, I agree that the inconsistency (empty string vs. NULL) is a bit confusing.

I do not mind too much either way - but if we allow both NULL and the
empty string to represent "no namespace", perhaps we should at least
document this in a comment next to the namespace field in struct
symbol and also next to the check in check_exports() so that it's
clear that both can mean no namespace.

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;