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* [PATCH] xen/x86: fix xen.efi boot crash from some bootloaders
@ 2025-07-23 13:56 Yann Sionneau
  2025-07-23 14:13 ` Andrew Cooper
                   ` (2 more replies)
  0 siblings, 3 replies; 11+ messages in thread
From: Yann Sionneau @ 2025-07-23 13:56 UTC (permalink / raw)
  To: xen-devel
  Cc: Yann Sionneau, Andrew Cooper, Anthony PERARD, Michal Orzel,
	Jan Beulich, Julien Grall, Roger Pau Monné,
	Stefano Stabellini

xen.efi PE does not boot when loaded from shim or some patched
downstream grub2.

What happens is the bootloader would honour the MEM_DISCARDABLE
flag of the .reloc section meaning it would not load its content
into memory.

But Xen is parsing the .reloc section content twice at boot:
* https://elixir.bootlin.com/xen/v4.20.1/source/xen/common/efi/boot.c#L1362
* https://elixir.bootlin.com/xen/v4.20.1/source/xen/arch/x86/efi/efi-boot.h#L237

Therefore it would crash with the following message:
"Unsupported relocation type" as reported there:

* https://github.com/QubesOS/qubes-issues/issues/8206#issuecomment-2619048838
* https://lore.kernel.org/xen-devel/7e039262-1f54-46e1-8f70-ac3f03607d5a@suse.com/T/#me122b9e6c27cd98db917da2c9f67e74a2c6ad7a5

This commit adds a small C host tool named keeprelocs
that is called after xen.efi is produced by the build system
in order to remove this bit from its .reloc section header.

Signed-off-by: Yann Sionneau <yann.sionneau@vates.tech>
---
 xen/Makefile           |   5 +-
 xen/arch/x86/Makefile  |   1 +
 xen/tools/Makefile     |   3 ++
 xen/tools/keeprelocs.c | 119 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 127 insertions(+), 1 deletion(-)
 create mode 100644 xen/tools/keeprelocs.c

diff --git a/xen/Makefile b/xen/Makefile
index 8fc4e042ff..7dc9cd7e05 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -299,10 +299,13 @@ export XEN_HAS_CHECKPOLICY := $(call success,$(CHECKPOLICY) -h 2>&1 | grep -q xe
 # ===========================================================================
 # Rules shared between *config targets and build targets
 
-PHONY += tools_fixdep
+PHONY += tools_fixdep tools_keeprelocs
 tools_fixdep:
 	$(Q)$(MAKE) $(build)=tools tools/fixdep
 
+tools_keeprelocs:
+	$(Q)$(MAKE) $(build)=tools tools/keeprelocs
+
 PHONY += outputmakefile
 # Before starting out-of-tree build, make sure the source tree is clean.
 # outputmakefile generates a Makefile in the output directory, if using a
diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index ce724a9daa..9a47002fae 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -236,6 +236,7 @@ endif
 	$(NM) -pa --format=sysv $@ \
 		| $(objtree)/tools/symbols --all-symbols --xensyms --sysv --sort \
 		> $@.map
+	$(objtree)/tools/keeprelocs -q -i $@
 ifeq ($(CONFIG_DEBUG_INFO),y)
 	$(if $(filter --strip-debug,$(EFI_LDFLAGS)),:$(space))$(OBJCOPY) -O elf64-x86-64 $@ $@.elf
 endif
diff --git a/xen/tools/Makefile b/xen/tools/Makefile
index a5078b7cb8..4fd917b398 100644
--- a/xen/tools/Makefile
+++ b/xen/tools/Makefile
@@ -1,2 +1,5 @@
 hostprogs-always-y += symbols
 hostprogs-always-y += fixdep
+hostprogs-always-$(XEN_BUILD_PE) += keeprelocs
+# next line is to allow including include/efi/pe.h
+HOSTCFLAGS_keeprelocs.o := -I ../include
\ No newline at end of file
diff --git a/xen/tools/keeprelocs.c b/xen/tools/keeprelocs.c
new file mode 100644
index 0000000000..c169ddba1a
--- /dev/null
+++ b/xen/tools/keeprelocs.c
@@ -0,0 +1,119 @@
+#include <stdio.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <efi/pe.h>
+
+#undef DEBUG
+
+static void print_usage(const char *name) {
+	printf("%s: [-q] [-h] -i xen.efi\n", name);
+}
+
+int main(int argc, char **argv)
+{
+	char *filename = NULL;
+	int fd;
+	char *mem;
+	struct stat st;
+	off_t len;
+	int ret;
+	struct mz_hdr *mz;
+	struct pe_hdr *pe;
+	int opt;
+	const char *prog_name = argv[0];
+	int quiet = 0;
+
+	while ((opt = getopt(argc, argv, ":i:qh")) != -1)
+	{
+		switch (opt) {
+		case 'i':
+			filename = optarg;
+			break;
+		case 'q':
+			quiet = 1;
+			break;
+		case 'h':
+			print_usage(prog_name);
+			return 0;
+			break;
+		case '?':
+		default:
+			print_usage(prog_name);
+			return -1;
+		}
+	}
+
+
+	if (!filename) {
+		printf("Error: you must provide a `-i xen.efi` argument\n");
+		return -1;
+	}
+
+	fd = open(filename, O_RDWR);
+	if (fd < 0) {
+		printf("Could not open file %s: %s\n", filename, strerror(errno));
+		return -1;
+	}
+
+	ret = fstat(fd, &st);
+	if (ret < 0) {
+		perror("Error while getting PE file length");
+		return -1;
+	}
+
+	len = st.st_size;
+	mem = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
+	if (mem == MAP_FAILED) {
+		perror("Failed to mmap PE file");
+		return -1;
+	}
+
+	mz = (struct mz_hdr *)mem;
+	if (mz->magic != MZ_MAGIC) { // "MZ"
+		printf("file has incorrect MZ header 0x%02x instead of 0x5a4d\n", mz->magic);
+		return -1;
+	}
+
+	pe = (struct pe_hdr *)(mem + mz->peaddr);
+	if (strncmp((char *)&pe->magic, "PE\0\0", 4)) {
+		printf("file has incorrect PE header magic %08x instead of 0x00004550\n", pe->magic);
+		return -1;
+	}
+
+	if (pe->opt_hdr_size == 0) {
+		printf("file has empty OptionalHeader\n");
+		return -1;
+	}
+
+	struct section_header *section = (struct section_header *)((uint8_t *)pe + sizeof(*pe) + pe->opt_hdr_size);
+	for (unsigned int section_id = 0; section_id < pe->sections; section_id++, section++)
+	{
+#ifdef DEBUG
+		printf("section %s\n", section->Name);
+#endif
+		if (strncmp(section->name, ".reloc", strlen(".reloc")))
+			continue;
+
+		if (!quiet)
+			printf(".reloc section characteristics: %08x\n", section->flags);
+		if (section->flags & IMAGE_SCN_MEM_DISCARDABLE) {
+			if (!quiet)
+				printf("MEM_DISCARDABLE flag found! Dropping it.\n");
+			section->flags &= ~(IMAGE_SCN_MEM_DISCARDABLE);
+		}
+	}
+
+	munmap(mem, len);
+	close(fd);
+
+	if (!quiet)
+		printf("Ok!\n");
+	return 0;
+}
-- 
2.43.0



Yann Sionneau | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech



^ permalink raw reply related	[flat|nested] 11+ messages in thread
end of thread, other threads:[~2025-07-24 14:11 UTC | newest]

Thread overview: 11+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2025-07-23 13:56 [PATCH] xen/x86: fix xen.efi boot crash from some bootloaders Yann Sionneau
2025-07-23 14:13 ` Andrew Cooper
2025-07-23 14:21   ` Jan Beulich
2025-07-23 14:38     ` Andrew Cooper
2025-07-23 14:45       ` Jan Beulich
2025-07-23 14:48         ` Jan Beulich
2025-07-23 14:18 ` Jan Beulich
2025-07-23 15:39   ` Yann Sionneau
2025-07-23 15:52     ` Jan Beulich
2025-07-24 14:11       ` Yann Sionneau
2025-07-24 12:18 ` Anthony PERARD

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