ipxe: Allow next loader path to be derived from shim path

Allow loader path to be constructed from the path used to load the
shim itself, e.g.:

    ipxe-shimx64.efi      -> ipxe.efi
    ipxe-shimaa64.efi     -> ipxe.efi
    snponly-shimx64.efi   -> snponly.efi
    snponly-shimaa64.efi  -> snponly.efi

This reduces the complexity of using a signed shim binary to load
iPXE, which (unlike GRUB) has a variety of possible binary names
depending on the requested driver set.  For example, if a site uses
all three of ipxe.efi, intel.efi, and snponly.efi then symlinks can be
used to provide the appropriate shim files:

    # iPXE binaries
    /var/lib/tftpboot/ipxe.efi
    /var/lib/tftpboot/intel.efi
    /var/lib/tftpboot/snponly.efi

    # shim binary (from this repository)
    /var/lib/tftpboot/ipxe-shimx64.efi

    # shim symlinks
    /var/lib/tftpboot/intel-shimx64.efi -> ipxe-shimx64.efi
    /var/lib/tftpboot/snponly-shimx64.efi -> ipxe-shimx64.efi

Unlike the shim binary names, iPXE binary names do not conventionally
include a CPU architecture suffix such as "x64" or "aa64": the
expectation is that if multiple architectures are in use at a site,
then there will be one directory per architecture.  For example:

    /var/lib/tftpboot/x86_64/ipxe.efi
    /var/lib/tftpboot/x86_64/ipxe-shimx64.efi
    /var/lib/tftpboot/arm64/ipxe.efi
    /var/lib/tftpboot/arm64/ipxe-shimaa64.efi

Signed-off-by: Michael Brown <[email protected]>

Author: mcb30

httpboot.c

@@ -175,6 +175,9 @@ generate_next_uri (CONST CHAR8 *current_uri, CONST CHAR8 *next_loader,
 		ptr++;
 	}
 
+	if((*uri = automatic_next_path(current_uri, 0, next_loader)))
+		return EFI_SUCCESS;
+
 	*uri = AllocatePool(sizeof(CHAR8) * (path_len + next_len + 1));
 	if (!*uri)
 		return EFI_OUT_OF_RESOURCES;

netboot.c

@@ -210,6 +210,10 @@ static BOOLEAN extract_tftp_info(CHAR8 *url, CHAR8 *name)
 		FreePool(template);
 		return FALSE;
 	}
+	if ((full_path = automatic_next_path(end, 0, name))) {
+		FreePool(template);
+		return TRUE;
+	}
 	full_path = AllocateZeroPool(strlen(end)+strlen(template)+1);
 	if (!full_path) {
 		FreePool(template);
@@ -276,6 +280,12 @@ static EFI_STATUS parseDhcp4(CHAR8 *name)
 	INTN i;
 	UINT8 *dir = pkt_v4->BootpBootFile;
 
+	if ((full_path = automatic_next_path((CHAR8 *)dir, dir_len, name))) {
+		memcpy(&tftp_addr.v4, pkt_v4->BootpSiAddr, 4);
+		FreePool(template);
+		return EFI_SUCCESS;
+	}
+
 	for (i = dir_len; i >= 0; i--) {
 		if ((dir[i] == '/') || (dir[i] == '\\'))
 			break;

shim.c

@@ -2004,3 +2004,59 @@ efi_main (EFI_HANDLE passed_image_handle, EFI_SYSTEM_TABLE *passed_systab)
 	devel_egress(EFI_ERROR(efi_status) ? EXIT_FAILURE : EXIT_SUCCESS);
 	return efi_status;
 }
+
+/*
+ * If we are attempting to download the default loader, then construct
+ * the path by changing "-shim[arch].efi" to ".efi", if possible.
+ *
+ * This code is maintained as a patch separate from the shim codebase,
+ * and so is deliberately optimised for minimal intrusiveness rather
+ * than elegance.
+ *
+ */
+CHAR8 *automatic_next_path(CONST CHAR8 *path, UINTN path_len,
+			   CONST CHAR8 *maybe_loader)
+{
+	CHAR8 *filename;
+	CHAR8 *hyphen;
+	CHAR8 *dot;
+	CHAR8 *next;
+
+	/* Check if this request is for the default loader */
+	if (strcmp(maybe_loader, DEFAULT_LOADER_CHAR) != 0) {
+		/* Not the default loader: use normal shim code path */
+		return NULL;
+	}
+
+	/* Copy and NUL-terminate the path */
+	if (!path_len)
+		path_len = strlen(path);
+	next = AllocateZeroPool(path_len + 1);
+	if (!next)
+		return NULL;
+	memcpy(next, path, path_len);
+	next[path_len] = '\0';
+
+	/* Locate filename portion of path */
+	filename = strrchr(next, '/');
+	if (filename) {
+		filename++;
+	} else {
+		filename = next;
+	}
+
+	/* Check for "-shim[arch].efi" suffix */
+	if ((hyphen = strrchr(filename, '-')) &&
+	    (strncasecmp(hyphen, "-shim", 5) == 0) &&
+	    (dot = strrchr(hyphen, '.')) &&
+	    (strcasecmp(dot, ".efi") == 0) &&
+	    (dot - hyphen <= 9)) {
+		/* Suffix is present: replace "-shim[arch]" with ".efi" */
+		strcpy(hyphen, dot);
+		return next;
+	}
+
+	/* Suffix not found: use normal shim code path */
+	FreePool(next);
+	return NULL;
+}

shim.h

@@ -324,4 +324,7 @@ verify_buffer (char *data, int datasize,
 
 char *translate_slashes(char *out, const char *str);
 
+extern CHAR8 *automatic_next_path(CONST CHAR8 *path, UINTN path_len,
+				  CONST CHAR8 *maybe_loader);
+
 #endif /* SHIM_H_ */