Messages in this thread | | | Date | Wed, 8 Dec 2021 11:19:52 -0800 | From | Kees Cook <> | Subject | Re: [PATCH] find: Do not read beyond variable boundaries on small sizes |
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On Tue, Dec 07, 2021 at 03:39:30PM -0800, Yury Norov wrote: > Bitmap functions work identically for all sizes from 0 to INT_MAX - 1. > Users don't 'choose between functions based on the size of their target'. > [...] > for_each_*_bit() requires a pointer to an array of unsigned longs. If > it's provided with something else, this is an error on a caller side.
I have a sense we're both talking past each other. Let me try to show what I'm seeing: the code in the bitmap API chooses one of two paths:
- if "size" is a constant expression and is sizeof(unsigned long) or smaller, mask to "size" and call into ff*() built-ins. - else, use a dynamic sized loop against a series of unsigned longs.
For the dynamic size case, yes, absolutely, things must stay unsigned long aligned, etc, that makes sense. I don't want to change anything there.
For the constant-expression size, this requirement does not hold. Every helper performs a constant expression sized mask of the argument before using ff*(), for example:
unsigned long val;
if (unlikely(offset >= size)) return size;
val = *addr & GENMASK(size - 1, offset); return val ? __ffs(val) : size;
In this case, the argument does _not_ need to be a pointer to native unsigned long. And this is seen in the many many places in the kernel using non-unsigned-long arguments with a constant-expression size. (e.g. an int with size 32).
-- Kees Cook
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