lib/PublicInbox/GitCatFile.pm | 10 +++++-----

diff --git a/lib/PublicInbox/GitCatFile.pm b/lib/PublicInbox/GitCatFile.pm
index 8bc6a238d2ae2c8f09c5062091e2ae140245d0c6..9bffce2aa2a727db21e1e42431dafc66310133c2 100644
--- a/lib/PublicInbox/GitCatFile.pm
+++ b/lib/PublicInbox/GitCatFile.pm
@@ -68,17 +68,17 @@ 		die "Unexpected result from git cat-file: $head\n";
 
 	my $size = $1;
 	my $bytes_left = $size;
-	my $buf;
+	my $offset = 0;
 	my $rv = '';
 
 	while ($bytes_left) {
-		my $read = read($in, $buf, $bytes_left);
-		defined($read) or die "read pipe failed: $!\n";
-		$rv .= $buf;
+		my $read = read($in, $rv, $bytes_left, $offset);
+		defined($read) or die "sysread pipe failed: $!\n";
 		$bytes_left -= $read;
+		$offset += $read;
 	}
 
-	my $read = read($in, $buf, 1);
+	my $read = read($in, my $buf, 1);
 	defined($read) or die "read pipe failed: $!\n";
 	if ($read != 1 || $buf ne "\n") {
 		die "newline missing after blob\n";