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
      */