Re: [PATCH] mm/fadvise: validate offset in generic_fadvise

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Dec 2025 15:18:17 +0100 klourencodev@gmail.com wrote:

> From: Kevin Lourenco <klourencodev@gmail.com>
> 
> When converted to (u64) for page calculations, a negative offset
> can produce extremely large page indices. This may lead to issues in certain advice modes (excessive readahead or
> cache invalidation)
> 
> offsets are normally non-negative, but the API does not guarantee this. Since 'len' is already
> validated, checking 'offset' here is reasonable to prevent potential system instability.
> 

Yeah, seems this code really didn't think about negative offsets.

	fadvise(fd, -20, 40, advice);

does make some sense I guess - equivalent to

	fadvise(fd, 0, 20, advice);

but whether this is how the implementation actually treats -20,40 is
unclear.  Probably not, and what will filemap_flush_range() make of
this.

ugh.

I suppose I'll toss this into -next for now, see if it breaks any tests. 
But more thought is needed.