[kvm-unit-tests PATCH v2 4/6] x86: efi_main: Self-relocate ELF .dynamic addresses

From: Varad Gautam <varadgautam@gmail.com>
To: Zixuan Wang <zixuanwang@google.com>,
	Nadav Amit <nadav.amit@gmail.com>, Marc Orr <marcorr@google.com>,
	Joerg Roedel <jroedel@suse.de>, kvm list <kvm@vger.kernel.org>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Andrew Jones <drjones@redhat.com>,
	bp@suse.de, Thomas.Lendacky@amd.com, brijesh.singh@amd.com,
	Hyunwook Baek <baekhw@google.com>,
	Erdem Aktas <erdemaktas@google.com>,
	Tom Roeder <tmroeder@google.com>
Cc: Varad Gautam <varad.gautam@suse.com>
Subject: [kvm-unit-tests PATCH v2 4/6] x86: efi_main: Self-relocate ELF .dynamic addresses
Date: Thu, 19 Aug 2021 13:33:58 +0200	[thread overview]
Message-ID: <20210819113400.26516-5-varad.gautam@suse.com> (raw)
In-Reply-To: <20210819113400.26516-1-varad.gautam@suse.com>

EFI expects a relocatable PE, and the loader will patch in the
relocations from the COFF.

Since we are wrapping an ELF into a PE here, the EFI loader will
not handle ELF relocations, and we need to patch the ELF .dynamic
section manually on early boot.

Signed-off-by: Varad Gautam <varad.gautam@suse.com>
---
 x86/efi_main.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/x86/efi_main.c b/x86/efi_main.c
index 237d4e7..be3f9ab 100644
--- a/x86/efi_main.c
+++ b/x86/efi_main.c
@@ -1,9 +1,13 @@
 #include <alloc_phys.h>
 #include <linux/uefi.h>
+#include <elf.h>
 
 unsigned long __efiapi efi_main(efi_handle_t handle, efi_system_table_t *sys_tab);
 efi_system_table_t *efi_system_table = NULL;
 
+extern char ImageBase;
+extern char _DYNAMIC;
+
 static void efi_free_pool(void *ptr)
 {
 	efi_bs_call(free_pool, ptr);
@@ -93,11 +97,70 @@ static efi_status_t exit_efi(void *handle)
 	return EFI_SUCCESS;
 }
 
+static efi_status_t elf_reloc(unsigned long image_base, unsigned long dynamic)
+{
+	long relsz = 0, relent = 0;
+	Elf64_Rel *rel = 0;
+	Elf64_Dyn *dyn = (Elf64_Dyn *) dynamic;
+	unsigned long *addr;
+	int i;
+
+	for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
+		switch (dyn[i].d_tag) {
+		case DT_RELA:
+			rel = (Elf64_Rel *)
+				((unsigned long) dyn[i].d_un.d_ptr + image_base);
+			break;
+		case DT_RELASZ:
+			relsz = dyn[i].d_un.d_val;
+			break;
+		case DT_RELAENT:
+			relent = dyn[i].d_un.d_val;
+			break;
+		default:
+			break;
+		}
+	}
+
+	if (!rel && relent == 0)
+		return EFI_SUCCESS;
+
+	if (!rel || relent == 0)
+		return EFI_LOAD_ERROR;
+
+	while (relsz > 0) {
+		/* apply the relocs */
+		switch (ELF64_R_TYPE (rel->r_info)) {
+		case R_X86_64_NONE:
+			break;
+		case R_X86_64_RELATIVE:
+			addr = (unsigned long *) (image_base + rel->r_offset);
+			*addr += image_base;
+			break;
+		default:
+			break;
+		}
+		rel = (Elf64_Rel *) ((char *) rel + relent);
+		relsz -= relent;
+	}
+	return EFI_SUCCESS;
+}
+
 unsigned long __efiapi efi_main(efi_handle_t handle, efi_system_table_t *sys_tab)
 {
+	unsigned long image_base, dyn;
 	efi_system_table = sys_tab;
 
 	exit_efi(handle);
 
+	image_base = (unsigned long) &ImageBase;
+	dyn = image_base + (unsigned long) &_DYNAMIC;
+
+	/* The EFI loader does not handle ELF relocations, so fixup
+	 * .dynamic addresses before proceeding any further. */
+	elf_reloc(image_base, dyn);
+
+	start64();
+
 	return 0;
 }
-- 
2.30.2

