195 lines
7.8 KiB
Diff
195 lines
7.8 KiB
Diff
From 7a3843972ea290daf1bec5e1133db654749b8c02 Mon Sep 17 00:00:00 2001
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From: Franck Bui <fbui@suse.com>
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Date: Tue, 22 Oct 2019 16:09:21 +0200
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Subject: [PATCH] fileio: introduce read_full_virtual_file() for reading
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virtual files in sysfs, procfs
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Virtual filesystems such as sysfs or procfs use kernfs, and kernfs can work
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with two sorts of virtual files.
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One sort uses "seq_file", and the results of the first read are buffered for
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the second read. The other sort uses "raw" reads which always go direct to the
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device.
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In the later case, the content of the virtual file must be retrieved with a
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single read otherwise subsequent read might get the new value instead of
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finding EOF immediately. That's the reason why the usage of fread(3) is
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prohibited in this case as it always performs a second call to read(2) looking
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for EOF which is subject to the race described previously.
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Fixes: #13585.
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(cherry picked from commit 21b40f16622f171a9969dc334d74fb5eb2f575c2)
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Related: #2117948
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---
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src/basic/fileio.c | 115 ++++++++++++++++++++++++++-
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src/basic/fileio.h | 1 +
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src/libsystemd/sd-device/sd-device.c | 2 +-
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3 files changed, 113 insertions(+), 5 deletions(-)
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diff --git a/src/basic/fileio.c b/src/basic/fileio.c
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index 6b0bad5b71..733fb42463 100644
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--- a/src/basic/fileio.c
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+++ b/src/basic/fileio.c
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@@ -276,6 +276,113 @@ int verify_file(const char *fn, const char *blob, bool accept_extra_nl) {
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return 1;
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}
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+int read_full_virtual_file(const char *filename, char **ret_contents, size_t *ret_size) {
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+ _cleanup_free_ char *buf = NULL;
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+ _cleanup_close_ int fd = -1;
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+ struct stat st;
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+ size_t n, size;
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+ int n_retries;
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+ char *p;
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+
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+ assert(ret_contents);
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+
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+ /* Virtual filesystems such as sysfs or procfs use kernfs, and kernfs can work
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+ * with two sorts of virtual files. One sort uses "seq_file", and the results of
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+ * the first read are buffered for the second read. The other sort uses "raw"
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+ * reads which always go direct to the device. In the latter case, the content of
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+ * the virtual file must be retrieved with a single read otherwise a second read
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+ * might get the new value instead of finding EOF immediately. That's the reason
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+ * why the usage of fread(3) is prohibited in this case as it always performs a
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+ * second call to read(2) looking for EOF. See issue 13585. */
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+
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+ fd = open(filename, O_RDONLY|O_CLOEXEC);
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+ if (fd < 0)
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+ return -errno;
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+
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+ /* Start size for files in /proc which usually report a file size of 0. */
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+ size = LINE_MAX / 2;
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+
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+ /* Limit the number of attempts to read the number of bytes returned by fstat(). */
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+ n_retries = 3;
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+
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+ for (;;) {
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+ if (n_retries <= 0)
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+ return -EIO;
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+
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+ if (fstat(fd, &st) < 0)
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+ return -errno;
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+
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+ if (!S_ISREG(st.st_mode))
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+ return -EBADF;
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+
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+ /* Be prepared for files from /proc which generally report a file size of 0. */
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+ if (st.st_size > 0) {
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+ size = st.st_size;
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+ n_retries--;
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+ } else
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+ size = size * 2;
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+
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+ if (size > READ_FULL_BYTES_MAX)
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+ return -E2BIG;
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+
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+ p = realloc(buf, size + 1);
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+ if (!p)
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+ return -ENOMEM;
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+ buf = TAKE_PTR(p);
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+
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+ for (;;) {
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+ ssize_t k;
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+
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+ /* Read one more byte so we can detect whether the content of the
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+ * file has already changed or the guessed size for files from /proc
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+ * wasn't large enough . */
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+ k = read(fd, buf, size + 1);
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+ if (k >= 0) {
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+ n = k;
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+ break;
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+ }
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+
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+ if (errno != -EINTR)
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+ return -errno;
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+ }
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+
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+ /* Consider a short read as EOF */
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+ if (n <= size)
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+ break;
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+
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+ /* Hmm... either we read too few bytes from /proc or less likely the content
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+ * of the file might have been changed (and is now bigger) while we were
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+ * processing, let's try again either with a bigger guessed size or the new
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+ * file size. */
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+
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+ if (lseek(fd, 0, SEEK_SET) < 0)
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+ return -errno;
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+ }
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+
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+ if (n < size) {
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+ p = realloc(buf, n + 1);
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+ if (!p)
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+ return -ENOMEM;
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+ buf = TAKE_PTR(p);
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+ }
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+
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+ if (!ret_size) {
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+ /* Safety check: if the caller doesn't want to know the size of what we
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+ * just read it will rely on the trailing NUL byte. But if there's an
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+ * embedded NUL byte, then we should refuse operation as otherwise
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+ * there'd be ambiguity about what we just read. */
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+
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+ if (memchr(buf, 0, n))
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+ return -EBADMSG;
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+ } else
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+ *ret_size = n;
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+
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+ buf[n] = 0;
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+ *ret_contents = TAKE_PTR(buf);
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+
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+ return 0;
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+}
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+
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int read_full_stream(FILE *f, char **contents, size_t *size) {
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_cleanup_free_ char *buf = NULL;
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struct stat st;
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@@ -300,9 +407,9 @@ int read_full_stream(FILE *f, char **contents, size_t *size) {
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if (st.st_size > READ_FULL_BYTES_MAX)
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return -E2BIG;
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- /* Start with the right file size, but be prepared for files from /proc which generally report a file
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- * size of 0. Note that we increase the size to read here by one, so that the first read attempt
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- * already makes us notice the EOF. */
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+ /* Start with the right file size. Note that we increase the size
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+ * to read here by one, so that the first read attempt already
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+ * makes us notice the EOF. */
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if (st.st_size > 0)
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n = st.st_size + 1;
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}
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@@ -986,7 +1093,7 @@ int get_proc_field(const char *filename, const char *pattern, const char *termin
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assert(pattern);
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assert(field);
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- r = read_full_file(filename, &status, NULL);
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+ r = read_full_virtual_file(filename, &status, NULL);
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if (r < 0)
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return r;
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diff --git a/src/basic/fileio.h b/src/basic/fileio.h
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index 77e6206e95..c6ad375b8d 100644
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--- a/src/basic/fileio.h
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+++ b/src/basic/fileio.h
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@@ -38,6 +38,7 @@ int write_string_filef(const char *fn, WriteStringFileFlags flags, const char *f
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int read_one_line_file(const char *fn, char **line);
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int read_full_file(const char *fn, char **contents, size_t *size);
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int read_full_stream(FILE *f, char **contents, size_t *size);
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+int read_full_virtual_file(const char *filename, char **ret_contents, size_t *ret_size);
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int verify_file(const char *fn, const char *blob, bool accept_extra_nl);
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diff --git a/src/libsystemd/sd-device/sd-device.c b/src/libsystemd/sd-device/sd-device.c
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index be29053f8c..49750ba9d7 100644
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--- a/src/libsystemd/sd-device/sd-device.c
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+++ b/src/libsystemd/sd-device/sd-device.c
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@@ -1798,7 +1798,7 @@ _public_ int sd_device_get_sysattr_value(sd_device *device, const char *sysattr,
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size_t size;
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/* read attribute value */
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- r = read_full_file(path, &value, &size);
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+ r = read_full_virtual_file(path, &value, &size);
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if (r < 0)
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return r;
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