From 9955c9456df52505b657b09b9caca1588b36f4ba Mon Sep 17 00:00:00 2001 From: Andreas Dilger Date: Thu, 4 Jul 2019 15:28:15 -0600 Subject: [PATCH] areadlink_with_size: handle size == 0 better If size is unset/zero when passed to areadlink_with_size(), the buffer allocation and while loop start out with buf_size = 1 and in most cases symlink length is considerably larger so there are several pointless loops while the buffer size is grown: $ strace -e readlink stat -c %N /proc/$$/cwd readlink("/proc/9036/cwd", "/", 1) = 1 readlink("/proc/9036/cwd", "/h", 2) = 2 readlink("/proc/9036/cwd", "/hom", 4) = 4 readlink("/proc/9036/cwd", "/home/pa", 8) = 8 readlink("/proc/9036/cwd", "/home/padraig", 16) = 13 Also, there is no point in allocating a buffer under 32 bytes. * src/areadlink-with-size.c: initialize buf_size=32 if size=0 --- lib/areadlink-with-size.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/lib/areadlink-with-size.c b/lib/areadlink-with-size.c index eacad3f..b49e05c 100644 --- a/lib/areadlink-with-size.c +++ b/lib/areadlink-with-size.c @@ -36,14 +36,15 @@ check, so it's OK to guess too small on hosts where there is no arbitrary limit to symbolic link length. */ #ifndef SYMLINK_MAX -# define SYMLINK_MAX 1024 +# define SYMLINK_MAX 1023 #endif #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX) /* Call readlink to get the symbolic link value of FILE. SIZE is a hint as to how long the link is expected to be; - typically it is taken from st_size. It need not be correct. + typically it is taken from st_size. It need not be correct, and a + value of 0 (or more than 8Ki) will select an appropriate lower bound. Return a pointer to that NUL-terminated string in malloc'd storage. If readlink fails, malloc fails, or if the link value is longer than SSIZE_MAX, return NULL (caller may use errno to diagnose). */ @@ -60,8 +61,10 @@ areadlink_with_size (char const *file, size_t size) ? symlink_max + 1 : INITIAL_LIMIT_BOUND); - /* The initial buffer size for the link value. */ - size_t buf_size = size < initial_limit ? size + 1 : initial_limit; + /* The initial buffer size for the link value. If size is unset, there is + no benefit to malloc() a buffer under 32 bytes, and it avoids several + iterations of the while loop compared to starting with size == 1. */ + size_t buf_size = size < initial_limit ? (size ?: 31) + 1 : initial_limit; while (1) { -- 2.4.5