Re: [PATCH v5 2/4] slab: Introduce kmalloc_obj() and family

[Matthew Wilcox <willy@infradead.org>]

On Mon, Nov 24, 2025 at 01:20:21PM -0800, Kees Cook wrote:
> > Maybe such a warning already exists and it's just too noisy to even
> > start thinking about turning it on?
> 
> Yes, -Wconversion (W=3) is mind-blowingly noisy, unfortunately.

It looks like GCC isn't smart enough.  The first warning I saw was legit
and easy to fix.  The second one is bogus:

include/linux/err.h: In function ‘PTR_ERR_OR_ZERO’:
include/linux/err.h:120:24: error: conversion from ‘long int’ to ‘int’ may change value [-Werror=conversion]
  120 |                 return PTR_ERR(ptr);

But GCC can prove that this isn't true; it just chooses not to:

#define IS_ERR_VALUE(x) unlikely((unsigned long)(void *)(x) >= (unsigned long)-MAX_ERRNO)

static inline bool __must_check IS_ERR(__force const void *ptr)
{
        return IS_ERR_VALUE((unsigned long)ptr);
}

static inline int __must_check PTR_ERR_OR_ZERO(__force const void *ptr)
{
        if (IS_ERR(ptr))
                return PTR_ERR(ptr);

So GCC knows in this path that 'ptr' is in the range [-4095..-1] and
the conversion from long to int will not change the value.

I imagine that fixing this is not high on the GCC developer priority
list, but if we filed a bug that might change?