linux-user: Fix access to /proc/self/exe

When accsssing /proc/self/exe from a userspace program, linux-user tries
to resolve the name via realpath(), which may fail if the process
changed the working directory in the meantime.

An example:
- a userspace program ist started with ./testprogram
- the program runs chdir("/tmp")
- then the program calls readlink("/proc/self/exe")
- linux-user tries to run realpath("./testprogram") which fails
  because ./testprogram isn't in /tmp
- readlink() will return -ENOENT back to the program

Avoid this issue by resolving the full path name of the started process
at startup of linux-user and store it in real_exec_path[]. This then
simplifies the emulation of readlink() and readlinkat() as well, because
they can simply copy the path string to userspace.

I noticed this bug because the testsuite of the debian package "pandoc"
failed on linux-user while it succeeded on real hardware.  The full log
is here:
https://buildd.debian.org/status/fetch.php?pkg=pandoc&arch=hppa&ver=2.17.1.1-1.1%2Bb1&stamp=1670153210&raw=0

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20221205113825.20615-1-deller@gmx.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/main.c

@@ -65,6 +65,7 @@
 #endif
 
 char *exec_path;
+char real_exec_path[PATH_MAX];
 
 int singlestep;
 static const char *argv0;
@@ -739,6 +740,11 @@ int main(int argc, char **argv, char **envp)
         }
     }
 
+    /* Resolve executable file name to full path name */
+    if (realpath(exec_path, real_exec_path)) {
+        exec_path = real_exec_path;
+    }
+
     /*
      * get binfmt_misc flags
      */

linux-user/syscall.c

@@ -9989,18 +9989,13 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
                 /* Short circuit this for the magic exe check. */
                 ret = -TARGET_EINVAL;
             } else if (is_proc_myself((const char *)p, "exe")) {
-                char real[PATH_MAX], *temp;
-                temp = realpath(exec_path, real);
-                /* Return value is # of bytes that we wrote to the buffer. */
-                if (temp == NULL) {
-                    ret = get_errno(-1);
-                } else {
-                    /* Don't worry about sign mismatch as earlier mapping
-                     * logic would have thrown a bad address error. */
-                    ret = MIN(strlen(real), arg3);
-                    /* We cannot NUL terminate the string. */
-                    memcpy(p2, real, ret);
-                }
+                /*
+                 * Don't worry about sign mismatch as earlier mapping
+                 * logic would have thrown a bad address error.
+                 */
+                ret = MIN(strlen(exec_path), arg3);
+                /* We cannot NUL terminate the string. */
+                memcpy(p2, exec_path, ret);
             } else {
                 ret = get_errno(readlink(path(p), p2, arg3));
             }
@@ -10021,18 +10016,13 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
                 /* Short circuit this for the magic exe check. */
                 ret = -TARGET_EINVAL;
             } else if (is_proc_myself((const char *)p, "exe")) {
-                char real[PATH_MAX], *temp;
-                temp = realpath(exec_path, real);
-                /* Return value is # of bytes that we wrote to the buffer. */
-                if (temp == NULL) {
-                    ret = get_errno(-1);
-                } else {
-                    /* Don't worry about sign mismatch as earlier mapping
-                     * logic would have thrown a bad address error. */
-                    ret = MIN(strlen(real), arg4);
-                    /* We cannot NUL terminate the string. */
-                    memcpy(p2, real, ret);
-                }
+                /*
+                 * Don't worry about sign mismatch as earlier mapping
+                 * logic would have thrown a bad address error.
+                 */
+                ret = MIN(strlen(exec_path), arg4);
+                /* We cannot NUL terminate the string. */
+                memcpy(p2, exec_path, ret);
             } else {
                 ret = get_errno(readlinkat(arg1, path(p), p2, arg4));
             }