* [PATCH v2 2/2] linux-user: Fix openat() emulation to not modify atime
2023-12-08 20:49 [PATCH v2 0/2] linux-user: openat() fixes Shu-Chun Weng
2023-12-08 20:49 ` [PATCH v2 1/2] linux-user: Define TARGET_O_LARGEFILE for aarch64 Shu-Chun Weng
@ 2023-12-08 20:49 ` Shu-Chun Weng
1 sibling, 0 replies; 3+ messages in thread
From: Shu-Chun Weng @ 2023-12-08 20:49 UTC (permalink / raw)
To: qemu-devel; +Cc: Laurent Vivier, Richard Henderson, Shu-Chun Weng
Commit b8002058 strengthened openat()'s /proc detection by calling
realpath(3) on the given path, which allows various paths and symlinks
that points to the /proc file system to be intercepted correctly.
Using realpath(3), though, has a side effect that it reads the symlinks
along the way, and thus changes their atime. The results in the
following code snippet already get ~now instead of the real atime:
int fd = open("/path/to/a/symlink", O_PATH | O_NOFOLLOW);
struct stat st;
fstat(fd, st);
return st.st_atime;
This change opens a path that doesn't appear to be part of /proc
directly and checks the destination of /proc/self/fd/n to determine if
it actually refers to a file in /proc.
Neither this nor the existing code works with symlinks or indirect paths
(e.g. /tmp/../proc/self/exe) that points to /proc/self/exe because it
is itself a symlink, and both realpath(3) and /proc/self/fd/n will
resolve into the location of QEMU.
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2004
Signed-off-by: Shu-Chun Weng <scw@google.com>
---
linux-user/syscall.c | 47 +++++++++++++++++++++++++++++++++++---------
1 file changed, 38 insertions(+), 9 deletions(-)
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index e384e14248..7c3772301f 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -8308,8 +8308,7 @@ static int open_net_route(CPUArchState *cpu_env, int fd)
int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
int flags, mode_t mode, bool safe)
{
- g_autofree char *proc_name = NULL;
- const char *pathname;
+ g_autofree char *pathname = NULL;
struct fake_open {
const char *filename;
int (*fill)(CPUArchState *cpu_env, int fd);
@@ -8334,12 +8333,42 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
{ NULL, NULL, NULL }
};
- /* if this is a file from /proc/ filesystem, expand full name */
- proc_name = realpath(fname, NULL);
- if (proc_name && strncmp(proc_name, "/proc/", 6) == 0) {
- pathname = proc_name;
+ if (strncmp(fname, "/proc/", 6) == 0) {
+ pathname = g_strdup(fname);
} else {
- pathname = fname;
+ g_autofree char *proc_name = NULL;
+ struct stat proc_stat;
+ int fd;
+
+ if (safe) {
+ fd = safe_openat(dirfd, path(fname), flags, mode);
+ } else {
+ fd = openat(dirfd, path(fname), flags, mode);
+ }
+ if (fd < 0) {
+ return fd;
+ }
+
+ /*
+ * Try to get the real path of the file we just opened. We avoid calling
+ * `realpath(3)` because it calls `readlink(2)` on symlinks which
+ * changes their atime. Note that since `/proc/self/exe` is a symlink,
+ * `pathname` will never resolve to it (neither will `realpath(3)`).
+ * That's why we check `fname` against the "/proc/" prefix first.
+ */
+ proc_name = g_strdup_printf("/proc/self/fd/%d", fd);
+ if (lstat(proc_name, &proc_stat) < 0 || !S_ISLNK(proc_stat.st_mode)) {
+ /* No procfs or something weird. Not going to dig further. */
+ return fd;
+ }
+ pathname = g_new(char, proc_stat.st_size + 1);
+ readlink(proc_name, pathname, proc_stat.st_size + 1);
+
+ /* if this is not a file from /proc/ filesystem, the fd is good as-is */
+ if (strncmp(pathname, "/proc/", 6) != 0) {
+ return fd;
+ }
+ close(fd);
}
if (is_proc_myself(pathname, "exe")) {
@@ -8390,9 +8419,9 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
}
if (safe) {
- return safe_openat(dirfd, path(pathname), flags, mode);
+ return safe_openat(dirfd, pathname, flags, mode);
} else {
- return openat(dirfd, path(pathname), flags, mode);
+ return openat(dirfd, pathname, flags, mode);
}
}
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