Re: [PATCH v3 2/3] sha1_file: open window into packfiles with O_CLOEXEC
- From
Junio C Hamano <gitster@pobox.com>
- Date
- Oct 28, 2016, 01:08 UTC
- Message-ID
- <xmqqa8dp46wx.fsf@gitster.mtv.corp.google.com>
- In-Reply-To
- <CA+55aFw83E+zOd+z5h-CA-3NhrLjVr-anL6pubrSWttYx3zu8g@mail.gmail.com>
Linus Torvalds <torvalds@linux-foundation.org> writes:
Show 8 quoted lines
> On Thu, Oct 27, 2016 at 4:36 PM, Junio C Hamano <gitster@pobox.com> wrote: >> >> Would the best endgame shape for this function be to open with >> O_NOATIME (and retry without), and then add CLOEXEC with fcntl(2) >> but ignoring an error from it, I guess? That would be the closest >> to what we historically had, I would think. > > I think that's the best model.
OK, so perhaps like this.
-- >8 -- Subject: git_open(): untangle possible NOATIME and CLOEXEC interactions
The way we structured the fallback-retry for opening with O_NOATIME and O_CLOEXEC meant that if we failed due to lack of support to open the file with O_NOATIME option (i.e. EINVAL), we would still try to drop O_CLOEXEC first and retry, and then drop O_NOATIME. A platform on which O_NOATIME is defined in the header without support from the kernel wouldn't have a chance to open with O_CLOEXEC option due to this code structure.
Arguably, O_CLOEXEC is more important than O_NOATIME, as the latter is mostly about performance, while the former can affect correctness. Let's revert the recent changes to the way git_open() attempts to open a file with O_NOATIME and retries without to the original sequence, and then use a separate fcntl(fd, F_SETFD, FD_CLOEXEC) on the resulting file descriptor. The helper to do the latter can be usable in the codepath in ce_compare_data() that was recently added to open a file descriptor with O_CLOEXEC, so let's refactor that codepath with the helper while we are at it.
Signed-off-by: Junio C Hamano <gitster@pobox.com> --- git-compat-util.h | 5 +++-- read-cache.c | 12 ++++-------- sha1_file.c | 49 ++++++++++++++++++++++++++++++------------------- 3 files changed, 37 insertions(+), 29 deletions(-)
diff --git a/git-compat-util.h b/git-compat-util.h index 43718dabae..a751630db5 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -679,9 +679,10 @@ char *gitstrdup(const char *s); #define getpagesize() sysconf(_SC_PAGESIZE) #endif -#ifndef O_CLOEXEC -#define O_CLOEXEC 0 +#ifndef FD_CLOEXEC +#define FD_CLOEXEC 0 #endif +extern int git_set_cloexec(int); #ifdef FREAD_READS_DIRECTORIES #ifdef fopen diff --git a/read-cache.c b/read-cache.c index db5d910642..fb91514885 100644 --- a/read-cache.c +++ b/read-cache.c @@ -156,17 +156,13 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st) static int ce_compare_data(const struct cache_entry *ce, struct stat *st) { int match = -1; - static int cloexec = O_CLOEXEC; - int fd = open(ce->name, O_RDONLY | cloexec); - - if ((cloexec & O_CLOEXEC) && fd < 0 && errno == EINVAL) { - /* Try again w/o O_CLOEXEC: the kernel might not support it */ - cloexec &= ~O_CLOEXEC; - fd = open(ce->name, O_RDONLY | cloexec); - } + int fd = open(ce->name, O_RDONLY); if (fd >= 0) { unsigned char sha1[20]; + + /* do not let child processes to hold onto the open fd */ + git_set_cloexec(fd); if (!index_fd(sha1, fd, st, OBJ_BLOB, ce->name, 0)) match = hashcmp(sha1, ce->oid.hash); /* index_fd() closed the file descriptor already */ diff --git a/sha1_file.c b/sha1_file.c index 09045df1dc..41383a6c20 100644 --- a/sha1_file.c +++ b/sha1_file.c @@ -1559,31 +1559,42 @@ int check_sha1_signature(const unsigned char *sha1, void *map, return hashcmp(sha1, real_sha1) ? -1 : 0; } -int git_open(const char *name) +int git_set_cloexec(int fd) { - static int sha1_file_open_flag = O_NOATIME | O_CLOEXEC; + static int cloexec = FD_CLOEXEC; - for (;;) { - int fd; + if (cloexec) { + if (fcntl(fd, F_SETFD, FD_CLOEXEC) < 0) + cloexec = 0; + /* + * We might want to diagnose and complain upon seeing + * an error from this call, but let's keep the same + * behaviour as before for now. + */ + } + return 0; +} - errno = 0; - fd = open(name, O_RDONLY | sha1_file_open_flag); - if (fd >= 0) - return fd; +int git_open(const char *name) +{ + static int noatime = O_NOATIME; + int fd; - /* Try again w/o O_CLOEXEC: the kernel might not support it */ - if ((sha1_file_open_flag & O_CLOEXEC) && errno == EINVAL) { - sha1_file_open_flag &= ~O_CLOEXEC; - continue; - } + errno = 0; + fd = open(name, O_RDONLY | noatime); - /* Might the failure be due to O_NOATIME? */ - if (errno != ENOENT && (sha1_file_open_flag & O_NOATIME)) { - sha1_file_open_flag &= ~O_NOATIME; - continue; - } - return -1; + /* Might the failure be due to O_NOATIME? */ + if ((noatime & O_NOATIME) && errno != ENOENT) { + noatime = 0; + fd = open(name, O_RDONLY); } + + if (fd < 0) + return fd; + + /* do not let child processes to hold onto the open fd */ + git_set_cloexec(fd); + return fd; } static int stat_sha1_file(const unsigned char *sha1, struct stat *st)