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The client runs as proot's root, which the kernel does not see, and leaves many content files read-only; an update that rewrites one was refused (EACCES) and cancelled with "Missing file permissions" (Proton Experimental (ARM64), wineopenxr.dll). For the client's own processes, a refused write-open of a file the app owns now gets the owner's write bit - on the file, or on the directory a new file goes into - and one more try. Games and Wine keep ordinary permissions.
77 lines
2.9 KiB
C
77 lines
2.9 KiB
C
/*
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* Root's way past a read-only file, for the Steam client.
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*
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* The client runs as root inside the session - proot's root, which the kernel never sees: to the
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* kernel it is the app's own user. The client leaves many content files read-only (r-x------,
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* ~290 files of Proton Experimental (ARM64) alone), and when an update later rewrites one of them
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* it opens the file for writing expecting root's permission to override the mode. The kernel
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* refuses (EACCES), the client reports "Missing file permissions" and cancels the update
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* (2026-09-26: Proton Experimental (ARM64), "CFileWriter: errno: 13 ... wineopenxr.dll").
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*
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* So for the client's own processes, a refused write-open of a file this user owns is answered the
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* way root's would be: the owner's write bit is added - to the file, or to the directory a new
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* file is being created in - and the open is tried once more. chmod only succeeds on what the app
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* owns, so nothing it could not already change is touched. Games, Wine and everything else keep
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* ordinary permissions: only the processes started from the client's own directories take this.
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*/
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#define _GNU_SOURCE
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#include <errno.h>
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#include <fcntl.h>
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#include <limits.h>
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#include <string.h>
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#include <sys/stat.h>
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#include <unistd.h>
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/* The client and its helpers run from the Steam root's platform directories. */
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static int is_steam_client(void) {
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static int cached = -1;
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char exe[PATH_MAX];
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ssize_t n;
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if (cached >= 0) return cached;
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n = readlink("/proc/self/exe", exe, sizeof(exe) - 1);
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cached = 0;
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if (n > 0) {
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exe[n] = '\0';
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cached = strstr(exe, "/Steam/steamrtarm64/") != NULL || strstr(exe, "/Steam/ubuntu12_64/") != NULL
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|| strstr(exe, "/Steam/linuxarm64/") != NULL;
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}
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return cached;
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}
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static int grant_owner_write(int dirfd, const char *path) {
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struct stat st;
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if (fstatat(dirfd, path, &st, 0) != 0 || (st.st_mode & S_IWUSR)) return 0;
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return fchmodat(dirfd, path, (st.st_mode & 07777) | S_IWUSR, 0) == 0;
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}
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/*
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* After an open refused with EACCES: returns 1 when a write bit was added and the open is worth
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* one more try. errno is left as the open set it.
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*/
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__attribute__((visibility("hidden")))
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int bl_writable_retry(int dirfd, const char *path, int flags) {
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int saved = errno;
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int granted = 0;
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char parent[PATH_MAX];
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const char *slash;
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if (saved != EACCES || path == NULL || (flags & O_ACCMODE) == O_RDONLY || geteuid() != 0 || !is_steam_client()) return 0;
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if (faccessat(dirfd, path, F_OK, 0) == 0) {
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granted = grant_owner_write(dirfd, path);
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} else if (flags & O_CREAT) {
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/* A new file in a read-only directory. */
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slash = strrchr(path, '/');
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if (slash == NULL) {
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granted = grant_owner_write(dirfd, ".");
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} else if ((size_t) (slash - path) < sizeof(parent)) {
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memcpy(parent, path, slash - path);
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parent[slash - path] = '\0';
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granted = grant_owner_write(dirfd, parent[0] ? parent : "/");
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}
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}
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errno = saved;
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return granted;
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}
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