Add ELF binfmt loader

Add binfmt loader for ELF. This is the default loader used on
execve().

Signed-off-by: Michalis Pappas <michalis@unikraft.io>

Makefile.uk

@@ -5,6 +5,7 @@ APPELFLOADER_CFLAGS-$(CONFIG_APPELFLOADER_DEBUG) += -DUK_DEBUG
 APPELFLOADER_SRCS-y += $(APPELFLOADER_BASE)/main.c
 APPELFLOADER_SRCS-y += $(APPELFLOADER_BASE)/elf_load.c
 APPELFLOADER_SRCS-y += $(APPELFLOADER_BASE)/elf_ctx.c
+APPELFLOADER_SRCS-y += $(APPELFLOADER_BASE)/elf_binfmt.c
 
 APPELFLOADER_SRCS-$(CONFIG_APPELFLOADER_BRK) += $(APPELFLOADER_BASE)/syscalls/brk.c
 UK_PROVIDED_SYSCALLS-$(CONFIG_APPELFLOADER_BRK) += brk-1

elf_binfmt.c

@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+/* Copyright (c) 2024, Unikraft GmbH and The Unikraft Authors.
+ * Licensed under the BSD-3-Clause License (the "License").
+ * You may not use this file except in compliance with the License.
+ */
+
+#include <uk/assert.h>
+#include <uk/arch/ctx.h>
+#include <uk/binfmt.h>
+#include <uk/essentials.h>
+#include <uk/errptr.h>
+#include <uk/init.h>
+
+#if CONFIG_LIBUKRANDOM
+#include <uk/random.h>
+#endif /* CONFIG_LIBUKRANDOM */
+
+#include "elf_prog.h"
+
+static int uk_binfmt_load_elf(struct uk_binfmt_loader_args *args)
+{
+	struct elf_prog *prog;
+	__u64 rand[2];
+	int rc;
+
+	UK_ASSERT(args);
+	UK_ASSERT(args->alloc);
+
+	/* TODO Make elf_load_vfs() modular so that we can check the file
+	 * type before we do the actual load. That will also allow us to
+	 * check the parameters before we load the file, as atm we're forced
+	 * to do an elf_unload() on bad parameters.
+	 */
+	prog = elf_load_vfs(args->alloc, args->pathname, args->progname);
+	if (unlikely(PTRISERR(prog))) {
+		rc = PTR2ERR(prog);
+		if (rc == -ENOEXEC) {
+			uk_pr_warn("%s not handled by ELF binfmt loader\n",
+				   args->pathname);
+			return UK_BINFMT_NOT_HANDLED;
+		}
+		uk_pr_err("Could not load ELF (%d)\n", rc);
+		return rc;
+	}
+
+	/* Save to private data in case we are requested to unload */
+	args->user = (void *)prog;
+
+	uk_pr_debug("%s: ELF loaded to 0x%lx-0x%lx (%lx B)\n", args->progname,
+		    (__u64)prog->vabase, (__u64)prog->vabase + prog->valen,
+		    prog->valen);
+	uk_pr_debug("%s: Entry at %p\n", args->progname, (void *)prog->entry);
+
+#if CONFIG_LIBUKRANDOM
+	uk_random_fill_buffer(rand, sizeof(rand));
+#else /* !CONFIG_LIBUKRANDOM */
+	/* Without random numbers, use a hardcoded seed */
+	uk_pr_warn("%s: Using hard-coded random seed\n", args->progname);
+	rand[0] = 0xB0B0;
+	rand[1] = 0xF00D;
+#endif /* !CONFIG_LIBUKRANDOM */
+
+	rc = elf_arg_env_count(&args->argc, args->argv,
+			       &args->envc, args->envp,
+			       args->stack_size);
+	if (unlikely(rc < 0)) {
+		uk_pr_err("Could not load ELF (%d)\n", rc);
+		elf_unload(prog);
+		return rc;
+	}
+
+	elf_ctx_init(&args->ctx, prog, args->progname,
+		     args->argc, args->argv,
+		     args->envc, args->envp, rand);
+
+	return UK_BINFMT_HANDLED;
+}
+
+static int uk_binfmt_unload_elf(struct uk_binfmt_loader_args *args)
+{
+	UK_ASSERT(args);
+	UK_ASSERT(args->user);
+
+	elf_unload((struct elf_prog *)args->user);
+
+	return UK_BINFMT_HANDLED;
+}
+
+static struct uk_binfmt_loader elf_loader = {
+	.name = "ELF loader",
+	.type = UK_BINFMT_LOADER_TYPE_EXEC,
+	.ops = {
+		.load = uk_binfmt_load_elf,
+		.unload = uk_binfmt_unload_elf,
+	},
+};
+
+static int uk_binfmt_elf_loader_init(struct uk_init_ctx *init_ctx __unused)
+{
+	return uk_binfmt_register(&elf_loader);
+}
+
+uk_late_initcall(uk_binfmt_elf_loader_init, 0);