efi/libstub: Fix path separator regression

Commit 9302c1bb ("efi/libstub: Rewrite file I/O routine") introduced a
regression that made a couple of (badly configured) systems fail to
boot [1]: Until 5.6, we silently accepted Unix-style file separators in
EFI paths, which might violate the EFI standard, but are an easy to make
mistake. This fix restores the pre-5.7 behaviour.

[1] https://bbs.archlinux.org/viewtopic.php?id=256273

Fixes: 9302c1bb ("efi/libstub: Rewrite file I/O routine")
Signed-off-by: Philipp Fent <fent@in.tum.de>
Link: https://lore.kernel.org/r/20200615115109.7823-1-fent@in.tum.de
[ardb: rewrite as chained if/else statements]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/file.c

@@ -102,12 +102,20 @@ static int find_file_option(const efi_char16_t *cmdline, int cmdline_len,
 	if (!found)
 		return 0;
 
+	/* Skip any leading slashes */
+	while (cmdline[i] == L'/' || cmdline[i] == L'\\')
+		i++;
+
 	while (--result_len > 0 && i < cmdline_len) {
-		if (cmdline[i] == L'\0' ||
-		    cmdline[i] == L'\n' ||
-		    cmdline[i] == L' ')
+		efi_char16_t c = cmdline[i++];
+
+		if (c == L'\0' || c == L'\n' || c == L' ')
 			break;
-		*result++ = cmdline[i++];
+		else if (c == L'/')
+			/* Replace UNIX dir separators with EFI standard ones */
+			*result++ = L'\\';
+		else
+			*result++ = c;
 	}
 	*result = L'\0';
 	return i;