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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include "alloc-util.h"
#include "copy.h"
#include "data-fd-util.h"
#include "fd-util.h"
#include "fs-util.h"
#include "io-util.h"
#include "memfd-util.h"
#include "tmpfile-util.h"
/* When the data is smaller or equal to 64K, try to place the copy in a memfd/pipe */
#define DATA_FD_MEMORY_LIMIT (64U*1024U)
/* If memfd/pipe didn't work out, then let's use a file in /tmp up to a size of 1M. If it's large than that use /var/tmp instead. */
#define DATA_FD_TMP_LIMIT (1024U*1024U)
int acquire_data_fd(const void *data, size_t size, unsigned flags) {
_cleanup_close_pair_ int pipefds[2] = { -1, -1 };
char pattern[] = "/dev/shm/data-fd-XXXXXX";
_cleanup_close_ int fd = -1;
int isz = 0, r;
ssize_t n;
off_t f;
assert(data || size == 0);
/* Acquire a read-only file descriptor that when read from returns the specified data. This is much more
* complex than I wish it was. But here's why:
*
* a) First we try to use memfds. They are the best option, as we can seal them nicely to make them
* read-only. Unfortunately they require kernel 3.17, and – at the time of writing – we still support 3.14.
*
* b) Then, we try classic pipes. They are the second best options, as we can close the writing side, retaining
* a nicely read-only fd in the reading side. However, they are by default quite small, and unprivileged
* clients can only bump their size to a system-wide limit, which might be quite low.
*
* c) Then, we try an O_TMPFILE file in /dev/shm (that dir is the only suitable one known to exist from
* earliest boot on). To make it read-only we open the fd a second time with O_RDONLY via
* /proc/self/<fd>. Unfortunately O_TMPFILE is not available on older kernels on tmpfs.
*
* d) Finally, we try creating a regular file in /dev/shm, which we then delete.
*
* It sucks a bit that depending on the situation we return very different objects here, but that's Linux I
* figure. */
if (size == 0 && ((flags & ACQUIRE_NO_DEV_NULL) == 0))
/* As a special case, return /dev/null if we have been called for an empty data block */
return RET_NERRNO(open("/dev/null", O_RDONLY|O_CLOEXEC|O_NOCTTY));
if ((flags & ACQUIRE_NO_MEMFD) == 0) {
fd = memfd_new("data-fd");
if (fd < 0)
goto try_pipe;
n = write(fd, data, size);
if (n < 0)
return -errno;
if ((size_t) n != size)
return -EIO;
f = lseek(fd, 0, SEEK_SET);
if (f != 0)
return -errno;
r = memfd_set_sealed(fd);
if (r < 0)
return r;
return TAKE_FD(fd);
}
try_pipe:
if ((flags & ACQUIRE_NO_PIPE) == 0) {
if (pipe2(pipefds, O_CLOEXEC|O_NONBLOCK) < 0)
return -errno;
isz = fcntl(pipefds[1], F_GETPIPE_SZ, 0);
if (isz < 0)
return -errno;
if ((size_t) isz < size) {
isz = (int) size;
if (isz < 0 || (size_t) isz != size)
return -E2BIG;
/* Try to bump the pipe size */
(void) fcntl(pipefds[1], F_SETPIPE_SZ, isz);
/* See if that worked */
isz = fcntl(pipefds[1], F_GETPIPE_SZ, 0);
if (isz < 0)
return -errno;
if ((size_t) isz < size)
goto try_dev_shm;
}
n = write(pipefds[1], data, size);
if (n < 0)
return -errno;
if ((size_t) n != size)
return -EIO;
(void) fd_nonblock(pipefds[0], false);
return TAKE_FD(pipefds[0]);
}
try_dev_shm:
if ((flags & ACQUIRE_NO_TMPFILE) == 0) {
fd = open("/dev/shm", O_RDWR|O_TMPFILE|O_CLOEXEC, 0500);
if (fd < 0)
goto try_dev_shm_without_o_tmpfile;
n = write(fd, data, size);
if (n < 0)
return -errno;
if ((size_t) n != size)
return -EIO;
/* Let's reopen the thing, in order to get an O_RDONLY fd for the original O_RDWR one */
return fd_reopen(fd, O_RDONLY|O_CLOEXEC);
}
try_dev_shm_without_o_tmpfile:
if ((flags & ACQUIRE_NO_REGULAR) == 0) {
fd = mkostemp_safe(pattern);
if (fd < 0)
return fd;
n = write(fd, data, size);
if (n < 0) {
r = -errno;
goto unlink_and_return;
}
if ((size_t) n != size) {
r = -EIO;
goto unlink_and_return;
}
/* Let's reopen the thing, in order to get an O_RDONLY fd for the original O_RDWR one */
r = open(pattern, O_RDONLY|O_CLOEXEC);
if (r < 0)
r = -errno;
unlink_and_return:
(void) unlink(pattern);
return r;
}
return -EOPNOTSUPP;
}
int copy_data_fd(int fd) {
_cleanup_close_ int copy_fd = -1, tmp_fd = -1;
_cleanup_free_ void *remains = NULL;
size_t remains_size = 0;
const char *td;
struct stat st;
int r;
/* Creates a 'data' fd from the specified source fd, containing all the same data in a read-only fashion, but
* independent of it (i.e. the source fd can be closed and unmounted after this call succeeded). Tries to be
* somewhat smart about where to place the data. In the best case uses a memfd(). If memfd() are not supported
* uses a pipe instead. For larger data will use an unlinked file in /tmp, and for even larger data one in
* /var/tmp. */
if (fstat(fd, &st) < 0)
return -errno;
/* For now, let's only accept regular files, sockets, pipes and char devices */
if (S_ISDIR(st.st_mode))
return -EISDIR;
if (S_ISLNK(st.st_mode))
return -ELOOP;
if (!S_ISREG(st.st_mode) && !S_ISSOCK(st.st_mode) && !S_ISFIFO(st.st_mode) && !S_ISCHR(st.st_mode))
return -EBADFD;
/* If we have reason to believe the data is bounded in size, then let's use memfds or pipes as backing fd. Note
* that we use the reported regular file size only as a hint, given that there are plenty special files in
* /proc and /sys which report a zero file size but can be read from. */
if (!S_ISREG(st.st_mode) || st.st_size < DATA_FD_MEMORY_LIMIT) {
/* Try a memfd first */
copy_fd = memfd_new("data-fd");
if (copy_fd >= 0) {
off_t f;
r = copy_bytes(fd, copy_fd, DATA_FD_MEMORY_LIMIT, 0);
if (r < 0)
return r;
f = lseek(copy_fd, 0, SEEK_SET);
if (f != 0)
return -errno;
if (r == 0) {
/* Did it fit into the limit? If so, we are done. */
r = memfd_set_sealed(copy_fd);
if (r < 0)
return r;
return TAKE_FD(copy_fd);
}
/* Hmm, pity, this didn't fit. Let's fall back to /tmp then, see below */
} else {
_cleanup_(close_pairp) int pipefds[2] = { -1, -1 };
int isz;
/* If memfds aren't available, use a pipe. Set O_NONBLOCK so that we will get EAGAIN rather
* then block indefinitely when we hit the pipe size limit */
if (pipe2(pipefds, O_CLOEXEC|O_NONBLOCK) < 0)
return -errno;
isz = fcntl(pipefds[1], F_GETPIPE_SZ, 0);
if (isz < 0)
return -errno;
/* Try to enlarge the pipe size if necessary */
if ((size_t) isz < DATA_FD_MEMORY_LIMIT) {
(void) fcntl(pipefds[1], F_SETPIPE_SZ, DATA_FD_MEMORY_LIMIT);
isz = fcntl(pipefds[1], F_GETPIPE_SZ, 0);
if (isz < 0)
return -errno;
}
if ((size_t) isz >= DATA_FD_MEMORY_LIMIT) {
r = copy_bytes_full(fd, pipefds[1], DATA_FD_MEMORY_LIMIT, 0, &remains, &remains_size, NULL, NULL);
if (r < 0 && r != -EAGAIN)
return r; /* If we get EAGAIN it could be because of the source or because of
* the destination fd, we can't know, as sendfile() and friends won't
* tell us. Hence, treat this as reason to fall back, just to be
* sure. */
if (r == 0) {
/* Everything fit in, yay! */
(void) fd_nonblock(pipefds[0], false);
return TAKE_FD(pipefds[0]);
}
/* Things didn't fit in. But we read data into the pipe, let's remember that, so that
* when writing the new file we incorporate this first. */
copy_fd = TAKE_FD(pipefds[0]);
}
}
}
/* If we have reason to believe this will fit fine in /tmp, then use that as first fallback. */
if ((!S_ISREG(st.st_mode) || st.st_size < DATA_FD_TMP_LIMIT) &&
(DATA_FD_MEMORY_LIMIT + remains_size) < DATA_FD_TMP_LIMIT) {
off_t f;
tmp_fd = open_tmpfile_unlinkable(NULL /* NULL as directory means /tmp */, O_RDWR|O_CLOEXEC);
if (tmp_fd < 0)
return tmp_fd;
if (copy_fd >= 0) {
/* If we tried a memfd/pipe first and it ended up being too large, then copy this into the
* temporary file first. */
r = copy_bytes(copy_fd, tmp_fd, UINT64_MAX, 0);
if (r < 0)
return r;
assert(r == 0);
}
if (remains_size > 0) {
/* If there were remaining bytes (i.e. read into memory, but not written out yet) from the
* failed copy operation, let's flush them out next. */
r = loop_write(tmp_fd, remains, remains_size, false);
if (r < 0)
return r;
}
r = copy_bytes(fd, tmp_fd, DATA_FD_TMP_LIMIT - DATA_FD_MEMORY_LIMIT - remains_size, COPY_REFLINK);
if (r < 0)
return r;
if (r == 0)
goto finish; /* Yay, it fit in */
/* It didn't fit in. Let's not forget to use what we already used */
f = lseek(tmp_fd, 0, SEEK_SET);
if (f != 0)
return -errno;
close_and_replace(copy_fd, tmp_fd);
remains = mfree(remains);
remains_size = 0;
}
/* As last fallback use /var/tmp */
r = var_tmp_dir(&td);
if (r < 0)
return r;
tmp_fd = open_tmpfile_unlinkable(td, O_RDWR|O_CLOEXEC);
if (tmp_fd < 0)
return tmp_fd;
if (copy_fd >= 0) {
/* If we tried a memfd/pipe first, or a file in /tmp, and it ended up being too large, than copy this
* into the temporary file first. */
r = copy_bytes(copy_fd, tmp_fd, UINT64_MAX, COPY_REFLINK);
if (r < 0)
return r;
assert(r == 0);
}
if (remains_size > 0) {
/* Then, copy in any read but not yet written bytes. */
r = loop_write(tmp_fd, remains, remains_size, false);
if (r < 0)
return r;
}
/* Copy in the rest */
r = copy_bytes(fd, tmp_fd, UINT64_MAX, COPY_REFLINK);
if (r < 0)
return r;
assert(r == 0);
finish:
/* Now convert the O_RDWR file descriptor into an O_RDONLY one (and as side effect seek to the beginning of the
* file again */
return fd_reopen(tmp_fd, O_RDONLY|O_CLOEXEC);
}
|