On Mon, Dec 4, 2023 at 8:58 AM Daniel P. Berrangé <berra...@redhat.com> wrote:
> On Thu, Nov 30, 2023 at 07:21:40PM -0800, Shu-Chun Weng wrote: > > 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. > > I wonder if we can detect that by opening with O_NOFOLLOW, then > calling fstatfs() on the FD, and checking f_type == PROCFS_SUPER_MAGIC > This works with indirect or relative paths, yes, but still not symlinks. I decided not to complicate the logic further. > > > > 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); > > g_strdup_printf() + g_autofree to avoid this PATH_MAX buffer > > > + n = readlink(procpath, pathname, sizeof(pathname)); > > + pathname[n < sizeof(pathname) ? n : sizeof(pathname)] = '\0'; > > If you call lstat() then sb_size will tell you how big the buffer > needs to be for a subsequent readlink(), whcih can be allocated > on the heap and released with g_autofree, avoiding the othuer PATH_MAX > buffer > Thanks for the suggestions, sent out v2 of the patch series eliminating both static buffers. Shu-Chun > > + > > + /* 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); > > } > > } > > > > > > With regards, > Daniel > -- > |: https://berrange.com -o- > https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- > https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- > https://www.instagram.com/dberrange :| > >
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