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loop: optionally use ioctl(LOOP_CONFIGURE) to set up loopdevs

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LOOP_CONFIGURE is added to Linux 5.8

function                                             old     new   delta
NO_LOOP_CONFIGURE (old code):
set_loop                                             784     782      -2
LOOP_CONFIGURE:
set_loop                                             784     653    -131
TRY_LOOP_CONFIGURE:
set_loop                                             784     811     +27

Based on a patch by Xiaoming Ni <nixiaoming@huawei.com>

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2022-12-13 14:27:08 +01:00
parent 9df54deead
commit 45734a2351
2 changed files with 65 additions and 9 deletions
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22
libbb/Config.src
View File

@ -369,3 +369,25 @@ config UNICODE_PRESERVE_BROKEN
For example, this means that entering 'l', 's', ' ', 0xff, [Enter]
at shell prompt will list file named 0xff (single char name
with char value 255), not file named '?'.
choice
prompt "Use LOOP_CONFIGURE for losetup and loop mounts"
default TRY_LOOP_CONFIGURE
help
LOOP_CONFIGURE is added to Linux 5.8
https://lwn.net/Articles/820408/
This allows userspace to completely setup a loop device with a single
ioctl, removing the in-between state where the device can be partially
configured - eg the loop device has a backing file associated with it,
but is reading from the wrong offset.
config LOOP_CONFIGURE
bool "use LOOP_CONFIGURE, needs kernel >= 5.8"
config NO_LOOP_CONFIGURE
bool "use LOOP_SET_FD + LOOP_SET_STATUS"
config TRY_LOOP_CONFIGURE
bool "try LOOP_CONFIGURE, fall back to LOOP_SET_FD + LOOP_SET_STATUS"
endchoice

52
libbb/loop.c
View File

@ -110,26 +110,39 @@ static int get_next_free_loop(char *dev, int id)
return loopdevno;
}
static int set_loopdev_params(int ffd,
int lfd, const char *file,
#if ENABLE_TRY_LOOP_CONFIGURE || ENABLE_LOOP_CONFIGURE
# define LOOP_CONFIGURE 0x4C0A
struct loop_config {
uint32_t fd;
uint32_t block_size;
struct loop_info64 info;
uint64_t __reserved[8];
};
#endif
static int set_loopdev_params(int lfd,
int ffd, const char *file,
unsigned long long offset,
unsigned long long sizelimit,
unsigned flags)
{
int rc;
#if ENABLE_TRY_LOOP_CONFIGURE || ENABLE_LOOP_CONFIGURE
struct loop_config lconfig;
# define loopinfo lconfig.info
#else
bb_loop_info loopinfo;
#endif
rc = ioctl(lfd, BB_LOOP_GET_STATUS, &loopinfo);
/* If device is free, try to claim it */
if (rc && errno == ENXIO) {
/* Associate free loop device with file */
rc = ioctl(lfd, LOOP_SET_FD, ffd);
if (rc != 0) {
/* Ouch... race: the device already has a fd */
return -1;
}
#if ENABLE_TRY_LOOP_CONFIGURE || ENABLE_LOOP_CONFIGURE
memset(&lconfig, 0, sizeof(lconfig));
#else
memset(&loopinfo, 0, sizeof(loopinfo));
#endif
safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
loopinfo.lo_offset = offset;
loopinfo.lo_sizelimit = sizelimit;
@ -140,6 +153,25 @@ static int set_loopdev_params(int ffd,
* is wrong (would free the loop device!)
*/
loopinfo.lo_flags = (flags & ~BB_LO_FLAGS_READ_ONLY);
#if ENABLE_TRY_LOOP_CONFIGURE || ENABLE_LOOP_CONFIGURE
lconfig.fd = ffd;
rc = ioctl(lfd, LOOP_CONFIGURE, &lconfig);
if (rc == 0)
return rc; /* SUCCESS! */
# if ENABLE_TRY_LOOP_CONFIGURE
if (errno != EINVAL)
return rc; /* error other than old kernel */
/* Old kernel, fall through into old way to do it: */
# endif
#endif
#if ENABLE_TRY_LOOP_CONFIGURE || ENABLE_NO_LOOP_CONFIGURE
/* Associate free loop device with file */
rc = ioctl(lfd, LOOP_SET_FD, ffd);
if (rc != 0) {
/* Ouch... race: the device already has a fd */
return rc;
}
rc = ioctl(lfd, BB_LOOP_SET_STATUS, &loopinfo);
if (rc != 0 && (loopinfo.lo_flags & BB_LO_FLAGS_AUTOCLEAR)) {
/* Old kernel, does not support LO_FLAGS_AUTOCLEAR? */
@ -151,8 +183,10 @@ static int set_loopdev_params(int ffd,
return rc; /* SUCCESS! */
/* failure, undo LOOP_SET_FD */
ioctl(lfd, LOOP_CLR_FD, 0); // actually, 0 param is unnecessary
#endif
}
return -1;
#undef loopinfo
}
/* Returns opened fd to the loop device, <0 on error.
@ -227,7 +261,7 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
goto try_next_loopN;
}
rc = set_loopdev_params(ffd, lfd, file, offset, sizelimit, flags);
rc = set_loopdev_params(lfd, ffd, file, offset, sizelimit, flags);
if (rc == 0) {
/* SUCCESS! */
if (!*device)