From 47f534bf92300c2e48c39999bc89e941ebc5d0c8 Mon Sep 17 00:00:00 2001 From: Michael Haggerty Date: Wed, 19 Jun 2013 08:36:26 +0200 Subject: [PATCH 1/3] resolve_ref_unsafe(): extract function handle_missing_loose_ref() The nesting was getting a bit out of hand, and it's about to get worse. Signed-off-by: Michael Haggerty Reviewed-by: Jeff King Signed-off-by: Junio C Hamano --- refs.c | 56 +++++++++++++++++++++++++++++++++++--------------------- 1 file changed, 35 insertions(+), 21 deletions(-) diff --git a/refs.c b/refs.c index 42a7e17f6bd..ab1f99ef9bc 100644 --- a/refs.c +++ b/refs.c @@ -1197,6 +1197,37 @@ static struct ref_entry *get_packed_ref(const char *refname) return find_ref(get_packed_refs(&ref_cache), refname); } +/* + * A loose ref file doesn't exist; check for a packed ref. The + * options are forwarded from resolve_safe_unsafe(). + */ +static const char *handle_missing_loose_ref(const char *refname, + unsigned char *sha1, + int reading, + int *flag) +{ + struct ref_entry *entry; + + /* + * The loose reference file does not exist; check for a packed + * reference. + */ + entry = get_packed_ref(refname); + if (entry) { + hashcpy(sha1, entry->u.value.sha1); + if (flag) + *flag |= REF_ISPACKED; + return refname; + } + /* The reference is not a packed reference, either. */ + if (reading) { + return NULL; + } else { + hashclr(sha1); + return refname; + } +} + const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int reading, int *flag) { int depth = MAXDEPTH; @@ -1222,28 +1253,11 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea git_snpath(path, sizeof(path), "%s", refname); if (lstat(path, &st) < 0) { - struct ref_entry *entry; - - if (errno != ENOENT) + if (errno == ENOENT) + return handle_missing_loose_ref(refname, sha1, + reading, flag); + else return NULL; - /* - * The loose reference file does not exist; - * check for a packed reference. - */ - entry = get_packed_ref(refname); - if (entry) { - hashcpy(sha1, entry->u.value.sha1); - if (flag) - *flag |= REF_ISPACKED; - return refname; - } - /* The reference is not a packed reference, either. */ - if (reading) { - return NULL; - } else { - hashclr(sha1); - return refname; - } } /* Follow "normalized" - ie "refs/.." symlinks by hand */ From 2884c06ae78e569a513701c2d43f4ed79bd252ce Mon Sep 17 00:00:00 2001 From: Michael Haggerty Date: Wed, 19 Jun 2013 08:36:27 +0200 Subject: [PATCH 2/3] resolve_ref_unsafe(): handle the case of an SHA-1 within loop There is only one "break" statement within the loop, which jumps to the code after the loop that handles the case of a file that holds a SHA-1. So move that code from below the loop into the if statement where the break was previously located. This makes the logic flow more local. Signed-off-by: Michael Haggerty Reviewed-by: Jeff King Signed-off-by: Junio C Hamano --- refs.c | 22 +++++++++++++--------- 1 file changed, 13 insertions(+), 9 deletions(-) diff --git a/refs.c b/refs.c index ab1f99ef9bc..867cf9f899b 100644 --- a/refs.c +++ b/refs.c @@ -1300,8 +1300,19 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea /* * Is it a symbolic ref? */ - if (prefixcmp(buffer, "ref:")) - break; + if (prefixcmp(buffer, "ref:")) { + /* + * Please note that FETCH_HEAD has a second + * line containing other data. + */ + if (get_sha1_hex(buffer, sha1) || + (buffer[40] != '\0' && !isspace(buffer[40]))) { + if (flag) + *flag |= REF_ISBROKEN; + return NULL; + } + return refname; + } if (flag) *flag |= REF_ISSYMREF; buf = buffer + 4; @@ -1314,13 +1325,6 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea } refname = strcpy(refname_buffer, buf); } - /* Please note that FETCH_HEAD has a second line containing other data. */ - if (get_sha1_hex(buffer, sha1) || (buffer[40] != '\0' && !isspace(buffer[40]))) { - if (flag) - *flag |= REF_ISBROKEN; - return NULL; - } - return refname; } char *resolve_refdup(const char *ref, unsigned char *sha1, int reading, int *flag) From fcb7c76274570e74fd9a9ac429095b07c2f34c64 Mon Sep 17 00:00:00 2001 From: Michael Haggerty Date: Wed, 19 Jun 2013 08:36:28 +0200 Subject: [PATCH 3/3] resolve_ref_unsafe(): close race condition reading loose refs We read loose references in two steps. The code is roughly: lstat() if error ENOENT: loose ref is missing; look for corresponding packed ref else if S_ISLNK: readlink() if error: report failure else if S_ISDIR: report failure else open() if error: report failure read() The problem is that the first filesystem call, to lstat(), is not atomic with the second filesystem call, to readlink() or open(). Therefore it is possible for another process to change the file between our two calls, for example: * If the other process deletes the file, our second call will fail with ENOENT, which we *should* interpret as "loose ref is missing; look for corresponding packed ref". This can arise if the other process is pack-refs; it might have just written a new packed-refs file containing the old contents of the reference then deleted the loose ref. * If the other process changes a symlink into a plain file, our call to readlink() will fail with EINVAL, which we *should* respond to by trying to open() and read() the file. The old code treats the reference as missing in both of these cases, which is incorrect. So instead, handle errors more selectively: if the result of readline()/open() is a failure that is inconsistent with the result of the previous lstat(), then something is fishy. In this case jump back and start over again with a fresh call to lstat(). One race is still possible and undetected: another process could change the file from a regular file into a symlink between the call to lstat and the call to open(). The open() call would silently follow the symlink and not know that something is wrong. This situation could be detected in two ways: * On systems that support O_NOFOLLOW, pass that option to the open(). * On other systems, call fstat() on the fd returned by open() and make sure that it agrees with the stat info from the original lstat(). However, we don't use symlinks anymore, so this situation is unlikely. Moreover, it doesn't appear that treating a symlink as a regular file would have grave consequences; after all, this is exactly how the code handles non-relative symlinks. So this commit leaves that race unaddressed. Note that this solves only the part of the race within resolve_ref_unsafe. In the situation described above, we may still be depending on a cached view of the packed-refs file; that race will be dealt with in a future patch. This problem was reported and diagnosed by Jeff King , and this solution is derived from his patch. Signed-off-by: Michael Haggerty Reviewed-by: Jeff King Signed-off-by: Junio C Hamano --- refs.c | 28 ++++++++++++++++++++++++---- 1 file changed, 24 insertions(+), 4 deletions(-) diff --git a/refs.c b/refs.c index 867cf9f899b..70b2c829fd1 100644 --- a/refs.c +++ b/refs.c @@ -1252,6 +1252,16 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea git_snpath(path, sizeof(path), "%s", refname); + /* + * We might have to loop back here to avoid a race + * condition: first we lstat() the file, then we try + * to read it as a link or as a file. But if somebody + * changes the type of the file (file <-> directory + * <-> symlink) between the lstat() and reading, then + * we don't want to report that as an error but rather + * try again starting with the lstat(). + */ + stat_ref: if (lstat(path, &st) < 0) { if (errno == ENOENT) return handle_missing_loose_ref(refname, sha1, @@ -1263,8 +1273,13 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea /* Follow "normalized" - ie "refs/.." symlinks by hand */ if (S_ISLNK(st.st_mode)) { len = readlink(path, buffer, sizeof(buffer)-1); - if (len < 0) - return NULL; + if (len < 0) { + if (errno == ENOENT || errno == EINVAL) + /* inconsistent with lstat; retry */ + goto stat_ref; + else + return NULL; + } buffer[len] = 0; if (!prefixcmp(buffer, "refs/") && !check_refname_format(buffer, 0)) { @@ -1287,8 +1302,13 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea * a ref */ fd = open(path, O_RDONLY); - if (fd < 0) - return NULL; + if (fd < 0) { + if (errno == ENOENT) + /* inconsistent with lstat; retry */ + goto stat_ref; + else + return NULL; + } len = read_in_full(fd, buffer, sizeof(buffer)-1); close(fd); if (len < 0)