On Thu, Oct 29, 2020 at 9:56 AM Christoph Hellwig hch@infradead.org wrote:

...

+int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,

• unsigned long *pfn)
  

The one tab indent here looks weird, normally tis would be two tabs or aligned aftetthe opening brace.

...

+{ +#ifdef CONFIG_STRICT_FOLLOW_PFN

• pr_info("unsafe follow_pfn usage rejected, see CONFIG_STRICT_FOLLOW_PFN\n");
  

• return -EINVAL;
  

+#else

• WARN_ONCE(1, "unsafe follow_pfn usage\n");
  

• add_taint(TAINT_USER, LOCKDEP_STILL_OK);
  

• return follow_pfn(vma, address, pfn);
  

+#endif

Woudn't this be a pretty good use case of "if (IS_ENABLED(...)))"?

Also I'd expect the inverse polarity of the config option, that is a USAFE_FOLLOW_PFN option to enable to unsafe behavior.

Was just about to send out v5, will apply your suggestions for that using IS_ENABLED.

Wrt negative or positive Kconfig, I was following STRICT_DEVMEM symbol as precedence. But easy to invert if there's strong feeling the other way round, I'm not attached to either.

...

+/**

• • unsafe_follow_pfn - look up PFN at a user virtual address
• • @vma: memory mapping
• • @address: user virtual address
• • @pfn: location to store found PFN
• • Only IO mappings and raw PFN mappings are allowed.
• • Returns zero and the pfn at @pfn on success, -ve otherwise.
• */

+int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,

• unsigned long *pfn)
  

+{

• return follow_pfn(vma, address, pfn);
  

+} +EXPORT_SYMBOL(unsafe_follow_pfn);

Any reason this doesn't use the warn and disable logic?

I figured without an mmu there's not much guarantees anyway. But I guess I can put it in here too for consistency. -Daniel