dir.c: reduce max pattern file size to 100MB

In a2bc523e1e (dir.c: skip .gitignore, etc larger than INT_MAX,
2024-05-31) we put capped the size of some files whose parsing code and
data structures used ints. Setting the limit to INT_MAX was a natural
spot, since we know the parsing code would misbehave above that.

But it also leaves the possibility of overflow errors when we multiply
that limit to allocate memory. For instance, a file consisting only of
"a\na\n..." could have INT_MAX/2 entries. Allocating an array of
pointers for each would need INT_MAX*4 bytes on a 64-bit system, enough
to overflow a 32-bit int.

So let's give ourselves a bit more safety margin by giving a much
smaller limit. The size 100MB is somewhat arbitrary, but is based on the
similar value for attribute files added by 3c50032ff5 (attr: ignore
overly large gitattributes files, 2022-12-01).

There's no particular reason these have to be the same, but the idea is
that they are in the ballpark of "so huge that nobody would care, but
small enough to avoid malicious overflow". So lacking a better guess, it
makes sense to use the same value. The implementation here doesn't share
the same constant, but we could change that later (or even give it a
runtime config knob, though nobody has complained yet about the
attribute limit).

And likewise, let's add a few tests that exercise the limits, based on
the attr ones. In this case, though, we never read .gitignore from the
index; the blob code is exercised only for sparse filters. So we'll
trigger it that way.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -32,6 +32,12 @@
 #include "tree.h"
 #include "hex.h"
 
+ /*
+  * The maximum size of a pattern/exclude file. If the file exceeds this size
+  * we will ignore it.
+  */
+#define PATTERN_MAX_FILE_SIZE (100 * 1024 * 1024)
+
 /*
  * Tells read_directory_recursive how a file or directory should be treated.
  * Values are ordered by significance, e.g. if a directory contains both
@@ -1137,7 +1143,7 @@ static int add_patterns(const char *fname, const char *base, int baselen,
 		}
 	}
 
-	if (size > INT_MAX) {
+	if (size > PATTERN_MAX_FILE_SIZE) {
 		warning("ignoring excessively large pattern file: %s", fname);
 		free(buf);
 		return -1;
@@ -1199,7 +1205,7 @@ int add_patterns_from_blob_to_list(
 	if (r != 1)
 		return r;
 
-	if (size > INT_MAX) {
+	if (size > PATTERN_MAX_FILE_SIZE) {
 		warning("ignoring excessively large pattern blob: %s",
 			oid_to_hex(oid));
 		free(buf);

t/t0008-ignores.sh

@@ -945,4 +945,12 @@ test_expect_success SYMLINKS 'symlinks not respected in-tree' '
 	test_grep "unable to access.*gitignore" err
 '
 
+test_expect_success EXPENSIVE 'large exclude file ignored in tree' '
+	test_when_finished "rm .gitignore" &&
+	dd if=/dev/zero of=.gitignore bs=101M count=1 &&
+	git ls-files -o --exclude-standard 2>err &&
+	echo "warning: ignoring excessively large pattern file: .gitignore" >expect &&
+	test_cmp expect err
+'
+
 test_done

t/t6112-rev-list-filters-objects.sh

@@ -701,4 +701,16 @@ test_expect_success 'expand blob limit in protocol' '
 	grep "blob:limit=1024" trace
 '
 
+test_expect_success EXPENSIVE 'large sparse filter file ignored' '
+	blob=$(dd if=/dev/zero bs=101M count=1 |
+	       git hash-object -w --stdin) &&
+	test_must_fail \
+		git rev-list --all --objects --filter=sparse:oid=$blob 2>err &&
+	cat >expect <<-EOF &&
+	warning: ignoring excessively large pattern blob: $blob
+	fatal: unable to parse sparse filter data in $blob
+	EOF
+	test_cmp expect err
+'
+
 test_done