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chase-symlinks: Add chase_symlinks_at()

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This commit is contained in:
Daan De Meyer 2022-09-25 09:18:41 +02:00
parent 15b4b4ed06
commit 5bc244aaa9
2 changed files with 194 additions and 120 deletions
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290
src/basic/chase-symlinks.c
View File

@ -57,15 +57,15 @@ static int log_autofs_mount_point(int fd, const char *path, ChaseSymlinksFlags f
strna(n1), path);
}
int chase_symlinks(
int chase_symlinks_at(
int dir_fd,
const char *path,
const char *original_root,
ChaseSymlinksFlags flags,
char **ret_path,
int *ret_fd) {
_cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL;
_cleanup_close_ int fd = -1;
_cleanup_free_ char *buffer = NULL, *done = NULL;
_cleanup_close_ int fd = -1, root_fd = -1;
unsigned max_follow = CHASE_SYMLINKS_MAX; /* how many symlinks to follow before giving up and returning ELOOP */
bool exists = true, append_trail_slash = false;
struct stat previous_stat;
@ -73,6 +73,8 @@ int chase_symlinks(
int r;
assert(path);
assert(!FLAGS_SET(flags, CHASE_PREFIX_ROOT));
assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
/* Either the file may be missing, or we return an fd to the final object, but both make no sense */
if ((flags & CHASE_NONEXISTENT) && ret_fd)
@ -82,27 +84,36 @@ int chase_symlinks(
return -EINVAL;
if (isempty(path))
return -EINVAL;
path = ".";
/* This is a lot like canonicalize_file_name(), but takes an additional "root" parameter, that allows following
* symlinks relative to a root directory, instead of the root of the host.
/* This function resolves symlinks of the path relative to the given directory file descriptor. If
* CHASE_SYMLINKS_RESOLVE_IN_ROOT is specified, symlinks are resolved relative to the given directory
* file descriptor. Otherwise, they are resolved relative to the root directory of the host.
*
* Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following
* relative symlinks to ensure they cannot be used to "escape" the root directory. The path parameter passed is
* assumed to be already prefixed by it, except if the CHASE_PREFIX_ROOT flag is set, in which case it is first
* prefixed accordingly.
* Note that when CHASE_SYMLINKS_RESOLVE_IN_ROOT is specified and we find an absolute symlink, it is
* resolved relative to given directory file descriptor and not the root of the host. Also, when
* following relative symlinks, this functions ensure they cannot be used to "escape" the given
* directory file descriptor. The "path" parameter is always interpreted relative to the given
* directory file descriptor. If the given directory file descriptor is AT_FDCWD and "path" is
* absolute, it is interpreted relative to the root directory of the host.
*
* Algorithmically this operates on two path buffers: "done" are the components of the path we already
* processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
* process. On each iteration, we move one component from "todo" to "done", processing it's special meaning
* each time. The "todo" path always starts with at least one slash, the "done" path always ends in no
* slash. We always keep an O_PATH fd to the component we are currently processing, thus keeping lookup races
* to a minimum.
* If "dir_fd" is a valid directory fd, "path" is an absolute path and "ret_path" is not NULL, this
* functions returns a relative path in "ret_path" because openat() like functions generally ignore
* the directory fd if they are provided with an absolute path. On the other hand, if "dir_fd" is
* AT_FDCWD and "path" is an absolute path, we need to return an absolute path in "ret_path" because
* otherwise, if the caller passes the returned relative path to another openat() like function, it
* would be resolved relative to the current working directory instead of to "/".
*
* Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute path you got
* as-is: fully qualified and relative to your host's root. Optionally, specify the root parameter to tell this
* function what to do when encountering a symlink with an absolute path as directory: prefix it by the
* specified path.
* Algorithmically this operates on two path buffers: "done" are the components of the path we
* already processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we
* still need to process. On each iteration, we move one component from "todo" to "done", processing
* it's special meaning each time. We always keep an O_PATH fd to the component we are currently
* processing, thus keeping lookup races to a minimum.
*
* Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute
* path you got as-is: fully qualified and relative to your host's root. Optionally, specify the
* "dir_fd" parameter to tell this function what to do when encountering a symlink with an absolute
* path as directory: resolve it relative to the given directory file descriptor.
*
* There are five ways to invoke this function:
*
@ -113,34 +124,33 @@ int chase_symlinks(
*
* 2. With ret_fd: in this case the destination is opened after chasing it as O_PATH and this file
* descriptor is returned as return value. This is useful to open files relative to some root
* directory. Note that the returned O_PATH file descriptors must be converted into a regular one (using
* fd_reopen() or such) before it can be used for reading/writing. ret_fd may not be combined with
* CHASE_NONEXISTENT.
* directory. Note that the returned O_PATH file descriptors must be converted into a regular one
* (using fd_reopen() or such) before it can be used for reading/writing. ret_fd may not be
* combined with CHASE_NONEXISTENT.
*
* 3. With CHASE_STEP: in this case only a single step of the normalization is executed, i.e. only the first
* symlink or ".." component of the path is resolved, and the resulting path is returned. This is useful if
* a caller wants to trace the path through the file system verbosely. Returns < 0 on error, > 0 if the
* path is fully normalized, and == 0 for each normalization step. This may be combined with
* CHASE_NONEXISTENT, in which case 1 is returned when a component is not found.
* 3. With CHASE_STEP: in this case only a single step of the normalization is executed, i.e. only
* the first symlink or ".." component of the path is resolved, and the resulting path is
* returned. This is useful if a caller wants to trace the path through the file system verbosely.
* Returns < 0 on error, > 0 if the path is fully normalized, and == 0 for each normalization
* step. This may be combined with CHASE_NONEXISTENT, in which case 1 is returned when a component
* is not found.
*
* 4. With CHASE_SAFE: in this case the path must not contain unsafe transitions, i.e. transitions from
* unprivileged to privileged files or directories. In such cases the return value is -ENOLINK. If
* CHASE_WARN is also set, a warning describing the unsafe transition is emitted. CHASE_WARN cannot
* be used in PID 1.
* 4. With CHASE_SAFE: in this case the path must not contain unsafe transitions, i.e. transitions
* from unprivileged to privileged files or directories. In such cases the return value is
* -ENOLINK. If CHASE_WARN is also set, a warning describing the unsafe transition is emitted.
* CHASE_WARN cannot be used in PID 1.
*
* 5. With CHASE_NO_AUTOFS: in this case if an autofs mount point is encountered, path normalization
* is aborted and -EREMOTE is returned. If CHASE_WARN is also set, a warning showing the path of
* the mount point is emitted. CHASE_WARN cannot be used in PID 1.
*/
/* A root directory of "/" or "" is identical to none */
if (empty_or_root(original_root))
original_root = NULL;
if (!(flags & (CHASE_AT_RESOLVE_IN_ROOT|CHASE_NONEXISTENT|CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_STEP)) &&
!ret_path && ret_fd) {
if (!original_root && !ret_path && !(flags & (CHASE_NONEXISTENT|CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_STEP)) && ret_fd) {
/* Shortcut the ret_fd case if the caller isn't interested in the actual path and has no root set
* and doesn't care about any of the other special features we provide either. */
r = open(path, O_PATH|O_CLOEXEC|((flags & CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
/* Shortcut the ret_fd case if the caller isn't interested in the actual path and has no root
* set and doesn't care about any of the other special features we provide either. */
r = openat(dir_fd, path, O_PATH|O_CLOEXEC|((flags & CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
if (r < 0)
return -errno;
@ -148,62 +158,38 @@ int chase_symlinks(
return 0;
}
if (original_root) {
r = path_make_absolute_cwd(original_root, &root);
if (r < 0)
return r;
buffer = strdup(path);
if (!buffer)
return -ENOMEM;
/* Simplify the root directory, so that it has no duplicate slashes and nothing at the
* end. While we won't resolve the root path we still simplify it. Note that dropping the
* trailing slash should not change behaviour, since when opening it we specify O_DIRECTORY
* anyway. Moreover at the end of this function after processing everything we'll always turn
* the empty string back to "/". */
delete_trailing_chars(root, "/");
path_simplify(root);
if (flags & CHASE_PREFIX_ROOT) {
buffer = path_join(root, path);
if (!buffer)
return -ENOMEM;
}
bool need_absolute = !FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT) && path_is_absolute(path);
if (need_absolute) {
done = strdup("/");
if (!done)
return -ENOMEM;
}
if (!buffer) {
r = path_make_absolute_cwd(path, &buffer);
if (r < 0)
return r;
}
if (FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT))
root_fd = openat(dir_fd, ".", O_CLOEXEC|O_DIRECTORY|O_PATH);
else
root_fd = open("/", O_CLOEXEC|O_DIRECTORY|O_PATH);
if (root_fd < 0)
return -errno;
fd = open(empty_to_root(root), O_CLOEXEC|O_DIRECTORY|O_PATH);
if (FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT) || !path_is_absolute(path))
fd = openat(dir_fd, ".", O_CLOEXEC|O_DIRECTORY|O_PATH);
else
fd = open("/", O_CLOEXEC|O_DIRECTORY|O_PATH);
if (fd < 0)
return -errno;
if (flags & CHASE_SAFE)
if (fstat(fd, &previous_stat) < 0)
return -errno;
if (fstat(fd, &previous_stat) < 0)
return -errno;
if (flags & CHASE_TRAIL_SLASH)
append_trail_slash = endswith(buffer, "/") || endswith(buffer, "/.");
if (root) {
/* If we are operating on a root directory, let's take the root directory as it is. */
todo = path_startswith(buffer, root);
if (!todo)
return log_full_errno(flags & CHASE_WARN ? LOG_WARNING : LOG_DEBUG,
SYNTHETIC_ERRNO(ECHRNG),
"Specified path '%s' is outside of specified root directory '%s', refusing to resolve.",
path, root);
done = strdup(root);
} else {
todo = buffer;
done = strdup("/");
}
if (!done)
return -ENOMEM;
for (;;) {
for (todo = buffer;;) {
_cleanup_free_ char *first = NULL;
_cleanup_close_ int child = -1;
struct stat st;
@ -228,34 +214,37 @@ int chase_symlinks(
_cleanup_free_ char *parent = NULL;
_cleanup_close_ int fd_parent = -1;
/* If we already are at the top, then going up will not change anything. This is in-line with
* how the kernel handles this. */
if (empty_or_root(done))
/* If we already are at the top, then going up will not change anything. This is
* in-line with how the kernel handles this. */
if (empty_or_root(done) && FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT))
continue;
fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH|O_DIRECTORY);
if (fd_parent < 0)
return -errno;
if (fstat(fd_parent, &st) < 0)
return -errno;
/* If we opened the same directory, that means we're at the host root directory, so
* going up won't change anything. */
if (st.st_dev == previous_stat.st_dev && st.st_ino == previous_stat.st_ino)
continue;
r = path_extract_directory(done, &parent);
if (r < 0)
if (r >= 0 || r == -EDESTADDRREQ)
free_and_replace(done, parent);
else if (IN_SET(r, -EINVAL, -EADDRNOTAVAIL)) {
/* If we're at the top of "dir_fd", start appending ".." to "done". */
if (!path_extend(&done, ".."))
return -ENOMEM;
} else
return r;
/* Don't allow this to leave the root dir. */
if (root &&
path_startswith(done, root) &&
!path_startswith(parent, root))
continue;
free_and_replace(done, parent);
if (flags & CHASE_STEP)
goto chased_one;
fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH);
if (fd_parent < 0)
return -errno;
if (flags & CHASE_SAFE) {
if (fstat(fd_parent, &st) < 0)
return -errno;
if (unsafe_transition(&previous_stat, &st))
return log_unsafe_transition(fd, fd_parent, path, flags);
@ -315,13 +304,13 @@ int chase_symlinks(
if (path_is_absolute(destination)) {
/* An absolute destination. Start the loop from the beginning, but use the root
* directory as base. */
/* An absolute destination. Start the loop from the beginning, but use the
* root file descriptor as base. */
safe_close(fd);
fd = open(empty_to_root(root), O_CLOEXEC|O_DIRECTORY|O_PATH);
fd = fd_reopen(root_fd, O_CLOEXEC|O_PATH|O_DIRECTORY);
if (fd < 0)
return -errno;
return fd;
if (flags & CHASE_SAFE) {
if (fstat(fd, &st) < 0)
@ -333,8 +322,7 @@ int chase_symlinks(
previous_stat = st;
}
/* Note that we do not revalidate the root, we take it as is. */
r = free_and_strdup(&done, empty_to_root(root));
r = free_and_strdup(&done, need_absolute ? "/" : NULL);
if (r < 0)
return r;
}
@ -401,6 +389,88 @@ chased_one:
return 0;
}
int chase_symlinks(
const char *path,
const char *original_root,
ChaseSymlinksFlags flags,
char **ret_path,
int *ret_fd) {
_cleanup_free_ char *root = NULL, *absolute = NULL, *p = NULL;
_cleanup_close_ int fd = -1, pfd = -1;
int r;
assert(path);
if (isempty(path))
return -EINVAL;
/* A root directory of "/" or "" is identical to none */
if (empty_or_root(original_root))
original_root = NULL;
if (original_root) {
r = path_make_absolute_cwd(original_root, &root);
if (r < 0)
return r;
/* Simplify the root directory, so that it has no duplicate slashes and nothing at the
* end. While we won't resolve the root path we still simplify it. Note that dropping the
* trailing slash should not change behaviour, since when opening it we specify O_DIRECTORY
* anyway. Moreover at the end of this function after processing everything we'll always turn
* the empty string back to "/". */
delete_trailing_chars(root, "/");
path_simplify(root);
if (flags & CHASE_PREFIX_ROOT) {
absolute = path_join(root, path);
if (!absolute)
return -ENOMEM;
}
}
if (!absolute) {
r = path_make_absolute_cwd(path, &absolute);
if (r < 0)
return r;
}
if (root) {
path = path_startswith(absolute, root);
if (!path)
return log_full_errno(flags & CHASE_WARN ? LOG_WARNING : LOG_DEBUG,
SYNTHETIC_ERRNO(ECHRNG),
"Specified path '%s' is outside of specified root directory '%s', refusing to resolve.",
absolute, root);
}
if (root) {
fd = open(root, O_CLOEXEC|O_DIRECTORY|O_PATH);
if (fd < 0)
return -errno;
flags |= CHASE_AT_RESOLVE_IN_ROOT;
} else
fd = AT_FDCWD;
r = chase_symlinks_at(fd, path, flags & ~CHASE_PREFIX_ROOT, ret_path ? &p : NULL, ret_fd ? &pfd : NULL);
if (r < 0)
return r;
if (ret_path) {
char *q = path_join(root, p);
if (!q)
return -ENOMEM;
*ret_path = TAKE_PTR(q);
}
if (ret_fd)
*ret_fd = TAKE_FD(pfd);
return r;
}
int chase_symlinks_and_open(
const char *path,
const char *root,

24
src/basic/chase-symlinks.h
View File

@ -7,16 +7,18 @@
#include "stat-util.h"
typedef enum ChaseSymlinksFlags {
CHASE_PREFIX_ROOT = 1 << 0, /* The specified path will be prefixed by the specified root before beginning the iteration */
CHASE_NONEXISTENT = 1 << 1, /* It's OK if the path doesn't actually exist. */
CHASE_NO_AUTOFS = 1 << 2, /* Return -EREMOTE if autofs mount point found */
CHASE_SAFE = 1 << 3, /* Return -EPERM if we ever traverse from unprivileged to privileged files or directories */
CHASE_TRAIL_SLASH = 1 << 4, /* Any trailing slash will be preserved */
CHASE_STEP = 1 << 5, /* Just execute a single step of the normalization */
CHASE_NOFOLLOW = 1 << 6, /* Do not follow the path's right-most component. With ret_fd, when the path's
* right-most component refers to symlink, return O_PATH fd of the symlink. */
CHASE_WARN = 1 << 7, /* Emit an appropriate warning when an error is encountered.
* Note: this may do an NSS lookup, hence this flag cannot be used in PID 1. */
CHASE_PREFIX_ROOT = 1 << 0, /* The specified path will be prefixed by the specified root before beginning the iteration */
CHASE_NONEXISTENT = 1 << 1, /* It's OK if the path doesn't actually exist. */
CHASE_NO_AUTOFS = 1 << 2, /* Return -EREMOTE if autofs mount point found */
CHASE_SAFE = 1 << 3, /* Return -EPERM if we ever traverse from unprivileged to privileged files or directories */
CHASE_TRAIL_SLASH = 1 << 4, /* Any trailing slash will be preserved */
CHASE_STEP = 1 << 5, /* Just execute a single step of the normalization */
CHASE_NOFOLLOW = 1 << 6, /* Do not follow the path's right-most component. With ret_fd, when the path's
* right-most component refers to symlink, return O_PATH fd of the symlink. */
CHASE_WARN = 1 << 7, /* Emit an appropriate warning when an error is encountered.
* Note: this may do an NSS lookup, hence this flag cannot be used in PID 1. */
CHASE_AT_RESOLVE_IN_ROOT = 1 << 8, /* Same as openat2()'s RESOLVE_IN_ROOT flag, symlinks are resolved
* relative to the given directory fd instead of root. */
} ChaseSymlinksFlags;
bool unsafe_transition(const struct stat *a, const struct stat *b);
@ -31,3 +33,5 @@ int chase_symlinks_and_opendir(const char *path, const char *root, ChaseSymlinks
int chase_symlinks_and_stat(const char *path, const char *root, ChaseSymlinksFlags chase_flags, char **ret_path, struct stat *ret_stat, int *ret_fd);
int chase_symlinks_and_access(const char *path, const char *root, ChaseSymlinksFlags chase_flags, int access_mode, char **ret_path, int *ret_fd);
int chase_symlinks_and_fopen_unlocked(const char *path, const char *root, ChaseSymlinksFlags chase_flags, const char *open_flags, char **ret_path, FILE **ret_file);
int chase_symlinks_at(int dir_fd, const char *path, ChaseSymlinksFlags flags, char **ret_path, int *ret_fd);