On Thu, Oct 29, 2020 at 9:56 AM Christoph Hellwig [off-list ref] wrote:

quoted

+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.

quoted

+{
+#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.

quoted

+/**
+ * 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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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