fs: binfmt_elf_efpic: add architecture hook elf_arch_finalize_exec

FDPIC ELF loader adds an architecture hook at the end of loading
binaries to finalize the mapped memory before moving toward exec
function.  The hook is used by UML under !MMU when translating
syscall/sysenter instructions before calling execve.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Kees Cook <kees@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

fs/binfmt_elf_fdpic.c

@@ -175,6 +175,12 @@ static int elf_fdpic_fetch_phdrs(struct elf_fdpic_params *params,
 	return 0;
 }
 
+int __weak elf_arch_finalize_exec(struct elf_fdpic_params *exec_params,
+				  struct elf_fdpic_params *interp_params)
+{
+	return 0;
+}
+
 /*****************************************************************************/
 /*
  * load an fdpic binary into various bits of memory
@@ -457,6 +463,10 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 			    dynaddr);
 #endif
 
+	retval = elf_arch_finalize_exec(&exec_params, &interp_params);
+	if (retval)
+		goto error;
+
 	finalize_exec(bprm);
 	/* everything is now ready... get the userspace context ready to roll */
 	entryaddr = interp_params.entry_addr ?: exec_params.entry_addr;

include/linux/elf-fdpic.h

@@ -56,4 +56,7 @@ extern void elf_fdpic_arch_lay_out_mm(struct elf_fdpic_params *exec_params,
 				      unsigned long *start_brk);
 #endif
 
+extern int elf_arch_finalize_exec(struct elf_fdpic_params *exec_params,
+				  struct elf_fdpic_params *interp_params);
+
 #endif /* _LINUX_ELF_FDPIC_H */