mm/fadvise: validate offset in generic_fadvise

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).

Reject negative offsets with -EINVAL for consistent argument validation
and to avoid silent misbehavior.

POSIX and the man page do not clearly define behavior for negative
offset/len.  FreeBSD rejects negative offsets as well, so failing with
-EINVAL is consistent with existing practice.  The man page can be updated
separately to document the Linux behavior.

Link: https://lkml.kernel.org/r/20260208135738.18992-1-klourencodev@gmail.com
Link: https://lkml.kernel.org/r/20251222141817.13335-1-klourencodev@gmail.com
Signed-off-by: Kevin Lourenco <k.lourenco@criteo.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/fadvise.c

@@ -43,7 +43,7 @@ int generic_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 		return -ESPIPE;
 
 	mapping = file->f_mapping;
-	if (!mapping || len < 0)
+	if (!mapping || len < 0 || offset < 0)
 		return -EINVAL;
 
 	bdi = inode_to_bdi(mapping->host);