Re: [PATCH] printk: avoid -Wsometimes-uninitialized warning

From: Arnd Bergmann
Date: Mon Sep 27 2021 - 09:28:44 EST

On Mon, Sep 27, 2021 at 3:20 PM Chris Down <chris@xxxxxxxxxxxxxx> wrote:
> Hi Arnd,
> Arnd Bergmann writes:
> >From: Arnd Bergmann <arnd@xxxxxxxx>
> >
> >clang notices that the pi_get_entry() function would use
> >uninitialized data if it was called with a non-NULL module
> >pointer on a kernel that does not support modules:
> On a !CONFIG_MODULES kernel, we _never_ pass a non-NULL module pointer. This
> isn't just convention: we don't even have `struct module` fully fleshed out, so
> it technically cannot be so.

Yes, I understand that part, hence the "if it was called" rather then
"when it is called".

> >kernel/printk/index.c:32:6: error: variable 'nr_entries' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> > if (!mod) {
> > ^~~~
> >kernel/printk/index.c:38:13: note: uninitialized use occurs here
> > if (pos >= nr_entries)
> > ^~~~~~~~~~
> >kernel/printk/index.c:32:2: note: remove the 'if' if its condition is always true
> > if (!mod) {
> >
> >Rework the condition to make it clear to the compiler that we are always
> >in the second case. Unfortunately the #ifdef is still required as the
> >definition of 'struct module' is hidden when modules are disabled.
> Having IS_ENABLED and then an #ifdef seems to hurt code readability to me.
> >Fixes: 337015573718 ("printk: Userspace format indexing support")
> Does this really fix anything, or just clang's ignorance? If the latter, clang
> needs to be smarter here: as far as I can see there are no occasions where
> there's even any opportunity for a non-NULL pointer to come in on a
> !CONFIG_MODULES kernel, since `struct module` isn't even complete.

I don't see how you would expect clang to understand that part. It does
not do cross-function analysis for the purpose of diagnostic output, and
even if it did, then this caller

static void *pi_next(struct seq_file *s, void *v, loff_t *pos)
const struct module *mod = s->file->f_inode->i_private;
struct pi_entry *entry = pi_get_entry(mod, *pos);

has no indication that "s->file->f_inode->i_private" is guaranteed to
be a NULL pointer.