Return ENOEXEC, not ENOENT, if a binary's or script's interpreter doesn't exist.

If you try to execute a binary compiled for ld-linux.so.1 (libc5) on a machine
with only ld-linux.so.2 (libc6), your shell will claim "mybinary: No such file
or directory", even though the binary exists. The ENOENT actually applies to
the ELF intepreter, not to the file itself. The same happens if you have a
nonexistent interpreter in a shell script's shebang line.

Give a more helpful and more expected error, "cannot execute binary file", in
these cases.

Signed-off-by: Geoffrey Thomas <geofft@mit.edu>
Tested-by: Anders Kaseorg <andersk@mit.edu>

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index b7c1603..56954e4 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -685,8 +685,11 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 
 			interpreter = open_exec(elf_interpreter);
 			retval = PTR_ERR(interpreter);
-			if (IS_ERR(interpreter))
+			if (IS_ERR(interpreter)) {
+				if (retval == -ENOENT)
+					retval = -ENOEXEC;
 				goto out_free_interp;
+			}
 
 			/*
 			 * If the binary is not readable then enforce
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 20fbece..604d117 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -232,6 +232,8 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
 			interpreter = open_exec(interpreter_name);
 			retval = PTR_ERR(interpreter);
 			if (IS_ERR(interpreter)) {
+				if (retval == -ENOENT)
+					retval = -ENOEXEC;
 				interpreter = NULL;
 				goto error;
 			}
diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index 32fb00b..587e0b9 100644
--- a/fs/binfmt_em86.c
+++ b/fs/binfmt_em86.c
@@ -82,8 +82,12 @@ static int load_em86(struct linux_binprm *bprm,struct pt_regs *regs)
 	 * space, and we don't need to copy it.
 	 */
 	file = open_exec(interp);
-	if (IS_ERR(file))
-		return PTR_ERR(file);
+	if (IS_ERR(file)) {
+		if (PTR_ERR(file) == -ENOENT)
+			return -ENOEXEC;
+		else
+			return PTR_ERR(file);
+	}
 
 	bprm->file = file;
 
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index c4e8353..eecf0db 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -180,8 +180,11 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 
 	interp_file = open_exec (iname);
 	retval = PTR_ERR (interp_file);
-	if (IS_ERR (interp_file))
+	if (IS_ERR (interp_file)) {
+		if (retval == -ENOENT)
+			retval = -ENOEXEC;
 		goto _error;
+	}
 
 	bprm->file = interp_file;
 	if (fmt->flags & MISC_FMT_CREDENTIALS) {
diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 0834350..738446f 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -88,8 +88,12 @@ static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)
 	 * OK, now restart the process with the interpreter's dentry.
 	 */
 	file = open_exec(interp);
-	if (IS_ERR(file))
-		return PTR_ERR(file);
+	if (IS_ERR(file)) {
+		if (PTR_ERR(file) == -ENOENT)
+			return -ENOEXEC;
+		else
+			return PTR_ERR(file);
+	}
 
 	bprm->file = file;
 	retval = prepare_binprm(bprm);