[PATCH v3 0/4] querying mount attributes
Miklos Szeredi
mszeredi at redhat.com
Thu Sep 28 13:01:42 UTC 2023
Implement mount querying syscalls agreed on at LSF/MM 2023.
Features:
- statx-like want/got mask
- allows returning ascii strings (fs type, root, mount point)
- returned buffer is relocatable (no pointers)
Still missing:
- man pages
- kselftest
- syscalls on non-x86 archs
Please find the test utility at the end of this mail.
Usage: statmnt [-l] (mnt_id|path)
Git tree:
git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git#statmount-v3
Changes v1..v3:
- rename statmnt(2) -> statmount(2)
- rename listmnt(2) -> listmount(2)
- make ABI 32bit compatible by passing 64bit args in a struct (tested on
i386 and x32)
- only accept new 64bit mount IDs
- fix compile on !CONFIG_PROC_FS
- call security_sb_statfs() in both syscalls
- make lookup_mnt_in_ns() static
- add LISTMOUNT_UNREACHABLE flag to listmnt() to explicitly ask for
listing unreachable mounts
- remove .sb_opts
- remove subtype from .fs_type
- return the number of bytes used (including strings) in .size
- rename .mountpoint -> .mnt_point
- point strings by an offset against char[] VLA at the end of the struct.
E.g. printf("fs_type: %s\n", st->str + st->fs_type);
- don't save string lengths
- extend spare space in struct statmnt (complete size is now 512 bytes)
---
Miklos Szeredi (4):
add unique mount ID
namespace: extract show_path() helper
add statmount(2) syscall
add listmount(2) syscall
arch/x86/entry/syscalls/syscall_32.tbl | 2 +
arch/x86/entry/syscalls/syscall_64.tbl | 2 +
fs/internal.h | 2 +
fs/mount.h | 3 +-
fs/namespace.c | 365 +++++++++++++++++++++++++
fs/proc_namespace.c | 10 +-
fs/stat.c | 9 +-
fs/statfs.c | 1 +
include/linux/syscalls.h | 8 +
include/uapi/asm-generic/unistd.h | 8 +-
include/uapi/linux/mount.h | 59 ++++
include/uapi/linux/stat.h | 1 +
12 files changed, 459 insertions(+), 11 deletions(-)
--
2.41.0
=== statmnt.c ===
#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/param.h>
#include <err.h>
/*
* Structure for getting mount/superblock/filesystem info with statmount(2).
*
* The interface is similar to statx(2): individual fields or groups can be
* selected with the @mask argument of statmount(). Kernel will set the @mask
* field according to the supported fields.
*
* If string fields are selected, then the caller needs to pass a buffer that
* has space after the fixed part of the structure. Nul terminated strings are
* copied there and offsets relative to @str are stored in the relevant fields.
* If the buffer is too small, then EOVERFLOW is returned. The actually used
* size is returned in @size.
*/
struct statmnt {
__u32 size; /* Total size, including strings */
__u32 __spare1;
__u64 mask; /* What results were written */
__u32 sb_dev_major; /* Device ID */
__u32 sb_dev_minor;
__u64 sb_magic; /* ..._SUPER_MAGIC */
__u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */
__u32 fs_type; /* [str] Filesystem type */
__u64 mnt_id; /* Unique ID of mount */
__u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */
__u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */
__u32 mnt_parent_id_old;
__u64 mnt_attr; /* MOUNT_ATTR_... */
__u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */
__u64 mnt_peer_group; /* ID of shared peer group */
__u64 mnt_master; /* Mount receives propagation from this ID */
__u64 propagate_from; /* Propagation from in current namespace */
__u32 mnt_root; /* [str] Root of mount relative to root of fs */
__u32 mnt_point; /* [str] Mountpoint relative to current root */
__u64 __spare2[50];
char str[]; /* Variable size part containing strings */
};
/*
* To be used on the kernel ABI only for passing 64bit arguments to statmount(2)
*/
struct __mount_arg {
__u64 mnt_id;
__u64 request_mask;
};
/*
* @mask bits for statmount(2)
*/
#define STMT_SB_BASIC 0x00000001U /* Want/got sb_... */
#define STMT_MNT_BASIC 0x00000002U /* Want/got mnt_... */
#define STMT_PROPAGATE_FROM 0x00000004U /* Want/got propagate_from */
#define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */
#define STMT_MNT_POINT 0x00000010U /* Want/got mnt_point */
#define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */
/* listmount(2) flags */
#define LISTMOUNT_UNREACHABLE 0x01 /* List unreachable mounts too */
#define __NR_statmount 454
#define __NR_listmount 455
#define STATX_MNT_ID_UNIQUE 0x00004000U /* Want/got extended stx_mount_id */
static void free_if_neq(void *p, const void *q)
{
if (p != q)
free(p);
}
static struct statmnt *statmount(uint64_t mnt_id, uint64_t mask, unsigned int flags)
{
struct __mount_arg arg = {
.mnt_id = mnt_id,
.request_mask = mask,
};
union {
struct statmnt m;
char s[4096];
} buf;
struct statmnt *ret, *mm = &buf.m;
size_t bufsize = sizeof(buf);
while (syscall(__NR_statmount, &arg, mm, bufsize, flags) == -1) {
free_if_neq(mm, &buf.m);
if (errno != EOVERFLOW)
return NULL;
bufsize = MAX(1 << 15, bufsize << 1);
mm = malloc(bufsize);
if (!mm)
return NULL;
}
ret = malloc(mm->size);
if (ret)
memcpy(ret, mm, mm->size);
free_if_neq(mm, &buf.m);
return ret;
}
static int listmount(uint64_t mnt_id, uint64_t **listp, unsigned int flags)
{
struct __mount_arg arg = {
.mnt_id = mnt_id,
};
uint64_t buf[512];
size_t bufsize = sizeof(buf);
uint64_t *ret, *ll = buf;
long len;
while ((len = syscall(__NR_listmount, &arg, ll, bufsize / sizeof(buf[0]), flags)) == -1) {
free_if_neq(ll, buf);
if (errno != EOVERFLOW)
return -1;
bufsize = MAX(1 << 15, bufsize << 1);
ll = malloc(bufsize);
if (!ll)
return -1;
}
bufsize = len * sizeof(buf[0]);
ret = malloc(bufsize);
if (!ret)
return -1;
*listp = ret;
memcpy(ret, ll, bufsize);
free_if_neq(ll, buf);
return len;
}
int main(int argc, char *argv[])
{
struct statmnt *st;
char *end;
const char *arg = argv[1];
int res;
int list = 0;
uint64_t mask = STMT_SB_BASIC | STMT_MNT_BASIC | STMT_PROPAGATE_FROM | STMT_MNT_ROOT | STMT_MNT_POINT | STMT_FS_TYPE;
uint64_t mnt_id;
if (arg && strcmp(arg, "-l") == 0) {
list = 1;
arg = argv[2];
}
if (argc != list + 2)
errx(1, "usage: %s [-l] (mnt_id|path)", argv[0]);
mnt_id = strtoll(arg, &end, 0);
if (!mnt_id || *end != '\0') {
struct statx sx;
res = statx(AT_FDCWD, arg, 0, STATX_MNT_ID_UNIQUE, &sx);
if (res == -1)
err(1, "%s", arg);
if (!(sx.stx_mask & (STATX_MNT_ID | STATX_MNT_ID_UNIQUE)))
errx(1, "Sorry, no mount ID");
mnt_id = sx.stx_mnt_id;
}
if (list) {
uint64_t *list;
int num, i;
res = listmount(mnt_id, &list, LISTMOUNT_UNREACHABLE);
if (res == -1)
err(1, "listmnt(%llu)", mnt_id);
num = res;
for (i = 0; i < num; i++) {
printf("0x%llx", list[i]);
st = statmount(list[i], STMT_MNT_POINT, 0);
if (!st) {
printf("\t[%s]\n", strerror(errno));
} else {
printf("\t%s\n", (st->mask & STMT_MNT_POINT) ? st->str + st->mnt_point : "???");
}
free(st);
}
free(list);
return 0;
}
st = statmount(mnt_id, mask, 0);
if (!st)
err(1, "statmnt(%llu)", mnt_id);
printf("size: %u\n", st->size);
printf("mask: 0x%llx\n", st->mask);
if (st->mask & STMT_SB_BASIC) {
printf("sb_dev_major: %u\n", st->sb_dev_major);
printf("sb_dev_minor: %u\n", st->sb_dev_minor);
printf("sb_magic: 0x%llx\n", st->sb_magic);
printf("sb_flags: 0x%08x\n", st->sb_flags);
}
if (st->mask & STMT_MNT_BASIC) {
printf("mnt_id: 0x%llx\n", st->mnt_id);
printf("mnt_parent_id: 0x%llx\n", st->mnt_parent_id);
printf("mnt_id_old: %u\n", st->mnt_id_old);
printf("mnt_parent_id_old: %u\n", st->mnt_parent_id_old);
printf("mnt_attr: 0x%08llx\n", st->mnt_attr);
printf("mnt_propagation: %s%s%s%s\n",
st->mnt_propagation & MS_SHARED ? "shared," : "",
st->mnt_propagation & MS_SLAVE ? "slave," : "",
st->mnt_propagation & MS_UNBINDABLE ? "unbindable," : "",
st->mnt_propagation & MS_PRIVATE ? "private" : "");
printf("mnt_peer_group: %llu\n", st->mnt_peer_group);
printf("mnt_master: %llu\n", st->mnt_master);
}
if (st->mask & STMT_PROPAGATE_FROM)
printf("propagate_from: %llu\n", st->propagate_from);
if (st->mask & STMT_MNT_ROOT)
printf("mnt_root: %u <%s>\n", st->mnt_root, st->str + st->mnt_root);
if (st->mask & STMT_MNT_POINT)
printf("mnt_point: %u <%s>\n", st->mnt_point, st->str + st->mnt_point);
if (st->mask & STMT_FS_TYPE)
printf("fs_type: %u <%s>\n", st->fs_type, st->str + st->fs_type);
free(st);
return 0;
}
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