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. Signed-off-by: Shu-Chun Weng <s...@google.com> --- linux-user/syscall.c | 42 +++++++++++++++++++++++++++++++++--------- 1 file changed, 33 insertions(+), 9 deletions(-) diff --git a/linux-user/syscall.c b/linux-user/syscall.c index e384e14248..25e2cda10a 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -8308,8 +8308,6 @@ 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; struct fake_open { const char *filename; int (*fill)(CPUArchState *cpu_env, int fd); @@ -8333,13 +8331,39 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname, #endif { NULL, NULL, NULL } }; + char pathname[PATH_MAX]; - /* 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) { + pstrcpy(pathname, sizeof(pathname), fname); } else { - pathname = fname; + char procpath[PATH_MAX]; + int fd, n; + + 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 resolves to it (neither will `realpath(3)`). + * That's why we check `fname` against the "/proc/" prefix first. + */ + snprintf(procpath, sizeof(procpath), "/proc/self/fd/%d", fd); + n = readlink(procpath, pathname, sizeof(pathname)); + pathname[n < sizeof(pathname) ? n : sizeof(pathname)] = '\0'; + + /* 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 +8414,9 @@ 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); } }