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mount: create loop devices with LO_FLAGS_AUTOCLEAR flag

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The "autolooped" mount (mount [-oloop] IMAGE /DIR/DIR)
always creates AUTOCLEARed loopdevs, so that umounting
drops them (and this does not require any code in the
umount userspace).
This happens since circa linux-2.6.25:
	commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
	Date:    Wed Feb 6 01:36:27 2008 -0800
	Subject: Allow auto-destruction of loop devices
IOW: in this case, umount does not have to use -d
to drop the loopdev.

The explicit loop mount (mount /dev/loopN /DIR/DIR)
does not do this. In this case, umount without -d
should not drop loopdev.
Unfortunately, bbox umount currently always implies -d,
this probably needs fixing.

function                                             old     new   delta
set_loop                                             537     597     +60
singlemount                                         1101    1138     +37
losetup_main                                         419     432     +13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 110/0)             Total: 110 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2017-03-16 16:49:37 +01:00
parent 018804204f
commit ab518eea9c
4 changed files with 87 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
include/libbb.h
View File

@ -1329,10 +1329,15 @@ extern int get_linux_version_code(void) FAST_FUNC;
extern char *query_loop(const char *device) FAST_FUNC;
extern int del_loop(const char *device) FAST_FUNC;
/* If *devname is not NULL, use that name, otherwise try to find free one,
/*
* If *devname is not NULL, use that name, otherwise try to find free one,
* malloc and return it in *devname.
* return value: 1: read-only loopdev was setup, 0: rw, < 0: error */
extern int set_loop(char **devname, const char *file, unsigned long long offset, int ro) FAST_FUNC;
* return value is the opened fd to the loop device, or < on error
*/
extern int set_loop(char **devname, const char *file, unsigned long long offset, unsigned flags) FAST_FUNC;
/* These constants match linux/loop.h (without BB_ prefix): */
#define BB_LO_FLAGS_READ_ONLY 1
#define BB_LO_FLAGS_AUTOCLEAR 4
/* Like bb_ask below, but asks on stdin with no timeout. */
char *bb_ask_stdin(const char * prompt) FAST_FUNC;

49
libbb/loop.c
View File

@ -78,22 +78,24 @@ int FAST_FUNC del_loop(const char *device)
return rc;
}
/* Returns 0 if mounted RW, 1 if mounted read-only, <0 for error.
*device is loop device to use, or if *device==NULL finds a loop device to
mount it on and sets *device to a strdup of that loop device name. This
search will re-use an existing loop device already bound to that
file/offset if it finds one.
/* Returns opened fd to the loop device, <0 on error.
* *device is loop device to use, or if *device==NULL finds a loop device to
* mount it on and sets *device to a strdup of that loop device name. This
* search will re-use an existing loop device already bound to that
* file/offset if it finds one.
*/
int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offset, int ro)
int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offset, unsigned flags)
{
char dev[LOOP_NAMESIZE];
char *try;
bb_loop_info loopinfo;
struct stat statbuf;
int i, dfd, ffd, mode, rc = -1;
int i, dfd, ffd, mode, rc;
rc = dfd = -1;
/* Open the file. Barf if this doesn't work. */
mode = ro ? O_RDONLY : O_RDWR;
mode = (flags & BB_LO_FLAGS_READ_ONLY) ? O_RDONLY : O_RDWR;
open_ffd:
ffd = open(file, mode);
if (ffd < 0) {
@ -144,20 +146,35 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
/* If device is free, claim it. */
if (rc && errno == ENXIO) {
memset(&loopinfo, 0, sizeof(loopinfo));
safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
loopinfo.lo_offset = offset;
/* Associate free loop device with file. */
if (ioctl(dfd, LOOP_SET_FD, ffd) == 0) {
if (ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo) == 0)
rc = 0;
else
memset(&loopinfo, 0, sizeof(loopinfo));
safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
loopinfo.lo_offset = offset;
/*
* Used by mount to set LO_FLAGS_AUTOCLEAR.
* LO_FLAGS_READ_ONLY is not set because RO is controlled by open type of the file.
* Note that closing LO_FLAGS_AUTOCLEARed dfd before mount
* is wrong (would free the loop device!)
*/
loopinfo.lo_flags = (flags & ~BB_LO_FLAGS_READ_ONLY);
rc = ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo);
if (rc != 0 && (loopinfo.lo_flags & BB_LO_FLAGS_AUTOCLEAR)) {
/* Old kernel, does not support LO_FLAGS_AUTOCLEAR? */
/* (this code path is not tested) */
loopinfo.lo_flags -= BB_LO_FLAGS_AUTOCLEAR;
rc = ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo);
}
if (rc != 0) {
ioctl(dfd, LOOP_CLR_FD, 0);
}
}
} else {
rc = -1;
}
close(dfd);
if (rc != 0) {
close(dfd);
}
try_again:
if (*device) break;
}
@ -165,7 +182,7 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
if (rc == 0) {
if (!*device)
*device = xstrdup(dev);
return (mode == O_RDONLY); /* 1:ro, 0:rw */
return dfd;
}
return rc;
}

27
util-linux/losetup.c
View File

@ -127,12 +127,37 @@ int losetup_main(int argc UNUSED_PARAM, char **argv)
d = *argv++;
if (argv[0]) {
if (set_loop(&d, argv[0], offset, (opt & OPT_r)) < 0)
if (set_loop(&d, argv[0], offset, (opt & OPT_r) ? BB_LO_FLAGS_READ_ONLY : 0) < 0)
bb_simple_perror_msg_and_die(argv[0]);
return EXIT_SUCCESS;
}
}
/* TODO: util-linux 2.28 shows this when run w/o params:
* NAME SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE DIO
* /dev/loop0 0 0 1 0 /PATH/TO/FILE 0
*
* implemented by reading /sys:
*
* open("/sys/block", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
* newfstatat(3, "loop0/loop/backing_file", {st_mode=S_IFREG|0444, st_size=4096, ...}, 0) = 0
* stat("/dev/loop0", {st_mode=S_IFBLK|0660, st_rdev=makedev(7, 0), ...}) = 0
* open("/sys/dev/block/7:0/loop/offset", O_RDONLY|O_CLOEXEC) = 5
* read(5, "0\n", 4096) = 2
* open("/sys/dev/block/7:0/loop/sizelimit", O_RDONLY|O_CLOEXEC) = 5
* read(5, "0\n", 4096) = 2
* open("/sys/dev/block/7:0/loop/offset", O_RDONLY|O_CLOEXEC) = 5
* read(5, "0\n", 4096) = 2
* open("/sys/dev/block/7:0/loop/autoclear", O_RDONLY|O_CLOEXEC) = 5
* read(5, "1\n", 4096) = 2
* open("/sys/dev/block/7:0/ro", O_RDONLY|O_CLOEXEC) = 5
* read(5, "0\n", 4096) = 2
* open("/sys/dev/block/7:0/loop/backing_file", O_RDONLY|O_CLOEXEC) = 5
* read(5, "/PATH/TO/FILE", 4096) = 37
* open("/sys/dev/block/7:0/loop/dio", O_RDONLY|O_CLOEXEC) = 5
* read(5, "0\n", 4096) = 2
*/
bb_show_usage(); /* does not return */
/*return EXIT_FAILURE;*/
}

21
util-linux/mount.c
View File

@ -1887,6 +1887,7 @@ static int nfsmount(struct mntent *mp, unsigned long vfsflags, char *filteropts)
// NB: mp->xxx fields may be trashed on exit
static int singlemount(struct mntent *mp, int ignore_busy)
{
int loopfd = -1;
int rc = -1;
unsigned long vfsflags;
char *loopFile = NULL, *filteropts = NULL;
@ -2026,7 +2027,20 @@ static int singlemount(struct mntent *mp, int ignore_busy)
if (ENABLE_FEATURE_MOUNT_LOOP && S_ISREG(st.st_mode)) {
loopFile = bb_simplify_path(mp->mnt_fsname);
mp->mnt_fsname = NULL; // will receive malloced loop dev name
if (set_loop(&mp->mnt_fsname, loopFile, 0, /*ro:*/ (vfsflags & MS_RDONLY)) < 0) {
// mount always creates AUTOCLEARed loopdevs, so that umounting
// drops them without any code in the userspace.
// This happens since circa linux-2.6.25:
// commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
// Date: Wed Feb 6 01:36:27 2008 -0800
// Subject: Allow auto-destruction of loop devices
loopfd = set_loop(&mp->mnt_fsname,
loopFile,
0,
((vfsflags & MS_RDONLY) ? BB_LO_FLAGS_READ_ONLY : 0)
| BB_LO_FLAGS_AUTOCLEAR
);
if (loopfd < 0) {
if (errno == EPERM || errno == EACCES)
bb_error_msg(bb_msg_perm_denied_are_you_root);
else
@ -2074,6 +2088,8 @@ static int singlemount(struct mntent *mp, int ignore_busy)
}
// If mount failed, clean up loop file (if any).
// (Newer kernels which support LO_FLAGS_AUTOCLEAR should not need this,
// merely "close(loopfd)" should do it?)
if (ENABLE_FEATURE_MOUNT_LOOP && rc && loopFile) {
del_loop(mp->mnt_fsname);
if (ENABLE_FEATURE_CLEAN_UP) {
@ -2086,6 +2102,9 @@ static int singlemount(struct mntent *mp, int ignore_busy)
if (ENABLE_FEATURE_CLEAN_UP)
free(filteropts);
if (loopfd >= 0)
close(loopfd);
if (errno == EBUSY && ignore_busy)
return 0;
if (errno == ENOENT && (vfsflags & MOUNT_NOFAIL))