[PATCH] file limit checking simplification

From: Hugh Dickins <hugh@veritas.com>

When handling rlimit != RLIM_INFINITY, generic_write_checks tests file
position against 0xFFFFFFFFULL, and casts it to a u32.  This code is
carried forward from 2.4.4, and the 2.4-ac tree contains an apparently
obvious fix to one part of it (should set count to 0 not to a negative).
But when you think it through, it all turns out to be bogus.

On a 32-bit architecture: limit is a 32-bit unsigned long, we've
already handled *pos < 0 and *pos >= limit, so *pos here has no way
of being > 0xFFFFFFFFULL, and thus casting it to u32 won't truncate it.
And on a 64-bit architecture: limit is a 64-bit unsigned long, but this
code is disallowing file position beyond the 32 bits; or if there's some
userspace compatibility issue, with limit having to fit into 32 bits,
the 32-bit architecture argument applies and they're still irrelevant.

So just remove the 0xFFFFFFFFULL test; and in place of the u32, cast to
typeof(limit) so it's right even if rlimits get wider.  And there's no
way we'd want to send SIGXFSZ below the limit: remove send_sig comment.

There's a similarly suspicious u32 cast a little further down, when
checking MAX_NON_LFS.  Given its definition, that does no harm on any
arch: but it's better changed to unsigned long, the type of MAX_NON_LFS.

mm/filemap.c

@@ -1509,9 +1509,8 @@ inline int generic_write_checks(struct inode *inode,
 				send_sig(SIGXFSZ, current, 0);
 				return -EFBIG;
 			}
-			if (*pos > 0xFFFFFFFFULL || *count > limit-(u32)*pos) {
-				/* send_sig(SIGXFSZ, current, 0); */
-				*count = limit - (u32)*pos;
+			if (*count > limit - (typeof(limit))*pos) {
+				*count = limit - (typeof(limit))*pos;
 			}
 		}
 	}
@@ -1525,9 +1524,8 @@ inline int generic_write_checks(struct inode *inode,
 			send_sig(SIGXFSZ, current, 0);
 			return -EFBIG;
 		}
-		if (*count > MAX_NON_LFS - (u32)*pos) {
-			/* send_sig(SIGXFSZ, current, 0); */
-			*count = MAX_NON_LFS - (u32)*pos;
+		if (*count > MAX_NON_LFS - (unsigned long)*pos) {
+			*count = MAX_NON_LFS - (unsigned long)*pos;
 		}
 	}