linux-user: Fix openat() emulation to correctly detect accesses to /proc

In qemu we catch accesses to files like /proc/cpuinfo or /proc/net/route
and return to the guest contents which would be visible on a real system
(instead what the host would show).

This patch fixes a bug, where for example the accesses
    cat /proc////cpuinfo
or
    cd /proc && cat cpuinfo
will not be recognized by qemu and where qemu will wrongly show
the contents of the host's /proc/cpuinfo file.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230803214450.647040-2-deller@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/syscall.c

@@ -8557,9 +8557,12 @@ static int open_hardware(CPUArchState *cpu_env, int fd)
 }
 #endif
 
-int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *pathname,
+
+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);
@@ -8585,6 +8588,14 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *pathname,
         { 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;
+    } else {
+        pathname = fname;
+    }
+
     if (is_proc_myself(pathname, "exe")) {
         if (safe) {
             return safe_openat(dirfd, exec_path, flags, mode);