[PATCH v4 25/34] build_id: Provide ld-embedded build-ids

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

This patch enables the Elf to be built with the build-id
and provide in the Xen hypervisor the code to extract it.

One can also retrieve the value of the build-id by doing
'readelf -n xen-syms'.

For EFI builds we re-use the same build-id that the xen-syms
was built with.

The version of ld that first implemented --build-id is v2.18.
Hence we check for that or later version - if older version
found we do not build the hypervisor with the build-id
(and the return code is -ENODATA for xen_build_id() call).

For x86 we have two binaries - the xen-syms and the xen - an
smaller version with lots of sections removed. To make it possible
for readelf -n xen we also modify mkelf32 and xen.lds.S to include
the PT_NOTE ELF section.

The EFI binary is more complicated. Having any non-recognizable
sections (.note, .data.note, etc) causes the boot to hang.
Moving the .note in the .data section makes it work. It is also
worth noting that the PE/COFF does not have any "comment"
sections to the author.

Lastly, we MUST call --binary-id=sha1 on all linker invocation so that
symbol offsets don't changes (which means we have multiple binary
ids - except that the last one is the final one). Without this change,
the symbol table embedded in Xen are incorrect - some of the values it
contains are offset by the size of the included build id.
This obviously causes problems when resolving symbols.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Martin Pohlack <mpohlack@amazon.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

---
Cc: Stefano Stabellini <stefano.stabellini@citrix.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>

v1: Rebase it on Martin's initial patch
v2: Move it to XENVER hypercall
v3: Fix EFI building (Ross's fix)
v4: Don't use the third argument for length.
v5: Use new structure for XENVER_build_id with variable buf.
v6: Include Ross's fix.
v7: Include detection of bin-utils for build-id support, add
    probing for size, and return -EPERM for XSM denied calls.
v8: Build xen_build_id under ARM, required adding ELFSIZE in proper file.
v9: Rebase on top XSM version class.
v10: Include the build-id .note in the xen ELF binary.
     s/build_id/build_id_linker/
    For EFI build, moved the --build-id values in .data section
v11: Rebase on staging.
v12: Split patch in two. Always do --build-id call. Include the .note in
    .rodata. USe const void * and ssize_t
v13: Use -S to make build_id.o and objcopy differently (Andrew suggested)
---
 Config.mk                   |  11 ++++
 xen/arch/arm/Makefile       |   2 +-
 xen/arch/arm/xen.lds.S      |  20 +++++--
 xen/arch/x86/Makefile       |  30 ++++++++--
 xen/arch/x86/boot/mkelf32.c | 137 ++++++++++++++++++++++++++++++++++++++------
 xen/arch/x86/xen.lds.S      |  23 ++++++++
 xen/common/version.c        |  51 +++++++++++++++++
 xen/include/xen/version.h   |   3 +
 8 files changed, 249 insertions(+), 28 deletions(-)

diff --git a/Config.mk b/Config.mk
index 79eb2bd..c8e89fe 100644
--- a/Config.mk
+++ b/Config.mk
@@ -126,6 +126,17 @@ endef
 check-$(gcc) = $(call cc-ver-check,CC,0x040100,"Xen requires at least gcc-4.1")
 $(eval $(check-y))
 
+ld-ver-build-id = $(shell $(1) --build-id 2>&1 | \
+					grep -q unrecognized && echo n || echo y)
+
+# binutils 2.18 implement build-id.
+ifeq ($(call ld-ver-build-id,$(LD)),n)
+build_id_linker :=
+else
+CFLAGS += -DBUILD_ID
+build_id_linker := --build-id=sha1
+endif
+
 # as-insn: Check whether assembler supports an instruction.
 # Usage: cflags-y += $(call as-insn "insn",option-yes,option-no)
 as-insn = $(if $(shell echo 'void _(void) { asm volatile ( $(2) ); }' \
diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
index 17e9e3a..a3319ab 100644
--- a/xen/arch/arm/Makefile
+++ b/xen/arch/arm/Makefile
@@ -94,7 +94,7 @@ $(TARGET)-syms: prelink.o xen.lds $(BASEDIR)/common/symbols-dummy.o
 	$(NM) -pa --format=sysv $(@D)/.$(@F).1 \
 		| $(BASEDIR)/tools/symbols --sysv --sort >$(@D)/.$(@F).1.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o $(build_id_linker) \
 	    $(@D)/.$(@F).1.o -o $@
 	rm -f $(@D)/.$(@F).[0-9]*
 
diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
index 9909595..187ef73 100644
--- a/xen/arch/arm/xen.lds.S
+++ b/xen/arch/arm/xen.lds.S
@@ -22,6 +22,9 @@ OUTPUT_ARCH(FORMAT)
 PHDRS
 {
   text PT_LOAD /* XXX should be AT ( XEN_PHYS_START ) */ ;
+#if defined(BUILD_ID)
+  note PT_NOTE ;
+#endif
 }
 SECTIONS
 {
@@ -50,16 +53,21 @@ SECTIONS
        __stop_bug_frames_2 = .;
        *(.rodata)
        *(.rodata.*)
-
-#ifdef LOCK_PROFILE
-       . = ALIGN(POINTER_ALIGN);
-       __lock_profile_start = .;
-       *(.lockprofile.data)
-       __lock_profile_end = .;
+#if !defined(BUILD_ID)
+        _erodata = .;          /* End of read-only data */
 #endif
+  } :text
 
+#if defined(BUILD_ID)
+  .note : {
+       __note_gnu_build_id_start = .;
+       *(.note.gnu.build-id)
+       __note_gnu_build_id_end = .;
+       *(.note)
+       *(.note.*)
         _erodata = .;          /* End of read-only data */
   } :text
+#endif
 
   .data : {                    /* Data */
        . = ALIGN(PAGE_SIZE);
diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 8a100be..7db2e53 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -71,9 +71,16 @@ efi-y := $(shell if [ ! -r $(BASEDIR)/include/xen/compile.h -o \
                       -O $(BASEDIR)/include/xen/compile.h ]; then \
                          echo '$(TARGET).efi'; fi)
 
+ifdef build_id_linker
+num_phdrs = 2
+else
+num_phdrs = 1
+endif
+
 $(TARGET): $(TARGET)-syms $(efi-y) boot/mkelf32
 	./boot/mkelf32 $(TARGET)-syms $(TARGET) 0x100000 \
-	`$(NM) -nr $(TARGET)-syms | head -n 1 | sed -e 's/^\([^ ]*\).*/0x\1/'`
+	`$(NM) -nr $(TARGET)-syms | head -n 1 | sed -e 's/^\([^ ]*\).*/0x\1/'` \
+	$(num_phdrs)
 	$(MAKE) -f $(BASEDIR)/Rules.mk -C test
 
 install:
@@ -109,22 +116,28 @@ $(BASEDIR)/common/symbols-dummy.o:
 	$(MAKE) -f $(BASEDIR)/Rules.mk -C $(BASEDIR)/common symbols-dummy.o
 
 $(TARGET)-syms: prelink.o xen.lds $(BASEDIR)/common/symbols-dummy.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o $(build_id_linker) \
 	    $(BASEDIR)/common/symbols-dummy.o -o $(@D)/.$(@F).0
 	$(NM) -pa --format=sysv $(@D)/.$(@F).0 \
 		| $(BASEDIR)/tools/symbols --all-symbols --sysv --sort \
 		>$(@D)/.$(@F).0.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).0.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o $(build_id_linker) \
 	    $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1
 	$(NM) -pa --format=sysv $(@D)/.$(@F).1 \
 		| $(BASEDIR)/tools/symbols --all-symbols --sysv --sort --warn-dup \
 		>$(@D)/.$(@F).1.S
 	$(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1.o
-	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o \
+	$(LD) $(LDFLAGS) -T xen.lds -N prelink.o $(build_id_linker) \
 	    $(@D)/.$(@F).1.o -o $@
 	rm -f $(@D)/.$(@F).[0-9]*
 
+build_id.o: $(TARGET)-syms
+	$(OBJCOPY) -O binary --only-section=.note $(BASEDIR)/xen-syms $@.bin
+	$(OBJCOPY) -I binary -O elf64-x86-64 -B i386:x86-64 \
+		--rename-section=.data=.note.gnu.build-id -S $@.bin $@
+	rm -f $@.bin
+
 EFI_LDFLAGS = $(patsubst -m%,-mi386pep,$(LDFLAGS)) --subsystem=10
 EFI_LDFLAGS += --image-base=$(1) --stack=0,0 --heap=0,0 --strip-debug
 EFI_LDFLAGS += --section-alignment=0x200000 --file-alignment=0x20
@@ -137,6 +150,13 @@ $(TARGET).efi: VIRT_BASE = 0x$(shell $(NM) efi/relocs-dummy.o | sed -n 's, A VIR
 $(TARGET).efi: ALT_BASE = 0x$(shell $(NM) efi/relocs-dummy.o | sed -n 's, A ALT_START$$,,p')
 # Don't use $(wildcard ...) here - at least make 3.80 expands this too early!
 $(TARGET).efi: guard = $(if $(shell echo efi/dis* | grep disabled),:)
+ifdef build_id_linker
+$(TARGET).efi: build_id.o
+build_id_file := build_id.o
+else
+build_id_file :=
+endif
+
 $(TARGET).efi: prelink-efi.o efi.lds efi/relocs-dummy.o $(BASEDIR)/common/symbols-dummy.o efi/mkreloc
 	$(foreach base, $(VIRT_BASE) $(ALT_BASE), \
 	          $(guard) $(LD) $(call EFI_LDFLAGS,$(base)) -T efi.lds -N $< efi/relocs-dummy.o \
@@ -153,7 +173,7 @@ $(TARGET).efi: prelink-efi.o efi.lds efi/relocs-dummy.o $(BASEDIR)/common/symbol
 		| $(guard) $(BASEDIR)/tools/symbols --sysv --sort >$(@D)/.$(@F).1s.S
 	$(guard) $(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1r.o $(@D)/.$(@F).1s.o
 	$(guard) $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T efi.lds -N $< \
-	                $(@D)/.$(@F).1r.o $(@D)/.$(@F).1s.o -o $@
+	                $(@D)/.$(@F).1r.o $(@D)/.$(@F).1s.o $(build_id_file) -o $@
 	if $(guard) false; then rm -f $@; echo 'EFI support disabled'; fi
 	rm -f $(@D)/.$(@F).[0-9]*
 
diff --git a/xen/arch/x86/boot/mkelf32.c b/xen/arch/x86/boot/mkelf32.c
index 993a7ee..d230e4c 100644
--- a/xen/arch/x86/boot/mkelf32.c
+++ b/xen/arch/x86/boot/mkelf32.c
@@ -45,9 +45,9 @@ static Elf32_Ehdr out_ehdr = {
     0,                                       /* e_flags */
     sizeof(Elf32_Ehdr),                      /* e_ehsize */
     sizeof(Elf32_Phdr),                      /* e_phentsize */
-    1,                                       /* e_phnum */
+    1,  /* modify based on num_phdrs */      /* e_phnum */
     sizeof(Elf32_Shdr),                      /* e_shentsize */
-    3,                                       /* e_shnum */
+    3,  /* modify based on num_phdrs */      /* e_shnum */
     2                                        /* e_shstrndx */
 };
 
@@ -61,8 +61,20 @@ static Elf32_Phdr out_phdr = {
     PF_R|PF_W|PF_X,                          /* p_flags */
     64                                       /* p_align */
 };
+static Elf32_Phdr note_phdr = {
+    PT_NOTE,                                 /* p_type */
+    DYNAMICALLY_FILLED,                      /* p_offset */
+    DYNAMICALLY_FILLED,                      /* p_vaddr */
+    DYNAMICALLY_FILLED,                      /* p_paddr */
+    DYNAMICALLY_FILLED,                      /* p_filesz */
+    DYNAMICALLY_FILLED,                      /* p_memsz */
+    PF_R,                                    /* p_flags */
+    4                                        /* p_align */
+};
 
 static u8 out_shstrtab[] = "\0.text\0.shstrtab";
+/* If num_phdrs >= 2, we need to tack the .note. */
+static u8 out_shstrtab_extra[] = ".note\0";
 
 static Elf32_Shdr out_shdr[] = {
     { 0 },
@@ -90,6 +102,23 @@ static Elf32_Shdr out_shdr[] = {
     }
 };
 
+/*
+ * The 17 points to the '.note' in the out_shstrtab and out_shstrtab_extra
+ * laid out in the file.
+ */
+static Elf32_Shdr out_shdr_extra = {
+      17,                                    /* sh_name */
+      SHT_NOTE,                              /* sh_type */
+      0,                                     /* sh_flags */
+      DYNAMICALLY_FILLED,                    /* sh_addr */
+      DYNAMICALLY_FILLED,                    /* sh_offset */
+      DYNAMICALLY_FILLED,                    /* sh_size */
+      0,                                     /* sh_link */
+      0,                                     /* sh_info */
+      4,                                     /* sh_addralign */
+      0                                      /* sh_entsize */
+};
+
 /* Some system header files define these macros and pollute our namespace. */
 #undef swap16
 #undef swap32
@@ -228,21 +257,22 @@ static void do_read(int fd, void *data, int len)
 int main(int argc, char **argv)
 {
     u64        final_exec_addr;
-    u32        loadbase, dat_siz, mem_siz;
+    u32        loadbase, dat_siz, mem_siz, note_base, note_sz, offset;
     char      *inimage, *outimage;
     int        infd, outfd;
     char       buffer[1024];
     int        bytes, todo, i;
+    int        num_phdrs;
 
     Elf32_Ehdr in32_ehdr;
 
     Elf64_Ehdr in64_ehdr;
     Elf64_Phdr in64_phdr;
 
-    if ( argc != 5 )
+    if ( argc != 6 )
     {
         fprintf(stderr, "Usage: mkelf32 <in-image> <out-image> "
-                "<load-base> <final-exec-addr>\n");
+                "<load-base> <final-exec-addr> <number of program headers>\n");
         return 1;
     }
 
@@ -250,7 +280,13 @@ int main(int argc, char **argv)
     outimage = argv[2];
     loadbase = strtoul(argv[3], NULL, 16);
     final_exec_addr = strtoull(argv[4], NULL, 16);
-
+    num_phdrs = atoi(argv[5]);
+    if ( num_phdrs > 2 || num_phdrs < 1 )
+    {
+        fprintf(stderr, "Number of program headers MUST be 1 or 2, got %d!\n",
+                num_phdrs);
+        return 1;
+    }
     infd = open(inimage, O_RDONLY);
     if ( infd == -1 )
     {
@@ -285,11 +321,10 @@ int main(int argc, char **argv)
                 (int)in64_ehdr.e_phentsize, (int)sizeof(in64_phdr));
         return 1;
     }
-
-    if ( in64_ehdr.e_phnum != 1 )
+    if ( in64_ehdr.e_phnum != num_phdrs )
     {
-        fprintf(stderr, "Expect precisly 1 program header; found %d.\n",
-                (int)in64_ehdr.e_phnum);
+        fprintf(stderr, "Expect precisly %d program header; found %d.\n",
+                num_phdrs, (int)in64_ehdr.e_phnum);
         return 1;
     }
 
@@ -299,11 +334,36 @@ int main(int argc, char **argv)
 
     (void)lseek(infd, in64_phdr.p_offset, SEEK_SET);
     dat_siz = (u32)in64_phdr.p_filesz;
-
     /* Do not use p_memsz: it does not include BSS alignment padding. */
     /*mem_siz = (u32)in64_phdr.p_memsz;*/
     mem_siz = (u32)(final_exec_addr - in64_phdr.p_vaddr);
 
+    note_sz = note_base = offset = 0;
+    if ( num_phdrs > 1 )
+    {
+        offset = in64_phdr.p_offset;
+        note_base = in64_phdr.p_vaddr;
+
+        (void)lseek(infd, in64_ehdr.e_phoff+sizeof(in64_phdr), SEEK_SET);
+        do_read(infd, &in64_phdr, sizeof(in64_phdr));
+        endianadjust_phdr64(&in64_phdr);
+
+        (void)lseek(infd, offset, SEEK_SET);
+
+        note_sz = in64_phdr.p_memsz;
+        note_base = in64_phdr.p_vaddr - note_base;
+
+        if ( in64_phdr.p_offset > dat_siz || offset > in64_phdr.p_offset )
+        {
+            fprintf(stderr, "Expected .note section within .text section!\n" \
+                    "Offset %ld not within %d!\n",
+                    in64_phdr.p_offset, dat_siz);
+            return 1;
+        }
+        /* Gets us the absolute offset within the .text section. */
+        offset = in64_phdr.p_offset - offset;
+    }
+
     /*
      * End the image on a page boundary. This gets round alignment bugs
      * in the boot- or chain-loader (e.g., kexec on the XenoBoot CD).
@@ -322,6 +382,31 @@ int main(int argc, char **argv)
     out_shdr[1].sh_size   = dat_siz;
     out_shdr[2].sh_offset = RAW_OFFSET + dat_siz + sizeof(out_shdr);
 
+    if ( num_phdrs > 1 )
+    {
+        /* We have two of them! */
+        out_ehdr.e_phnum = num_phdrs;
+        /* Extra .note section. */
+        out_ehdr.e_shnum++;
+
+        /* Fill out the PT_NOTE program header. */
+        note_phdr.p_vaddr   = note_base;
+        note_phdr.p_paddr   = note_base;
+        note_phdr.p_filesz  = note_sz;
+        note_phdr.p_memsz   = note_sz;
+        note_phdr.p_offset  = offset;
+
+        /* Tack on the .note\0 */
+        out_shdr[2].sh_size += sizeof(out_shstrtab_extra);
+        /* And move it past the .note section. */
+        out_shdr[2].sh_offset += sizeof(out_shdr_extra);
+
+        /* Fill out the .note section. */
+        out_shdr_extra.sh_size = note_sz;
+        out_shdr_extra.sh_addr = note_base;
+        out_shdr_extra.sh_offset = RAW_OFFSET + offset;
+    }
+
     outfd = open(outimage, O_WRONLY|O_CREAT|O_TRUNC, 0775);
     if ( outfd == -1 )
     {
@@ -335,8 +420,15 @@ int main(int argc, char **argv)
 
     endianadjust_phdr32(&out_phdr);
     do_write(outfd, &out_phdr, sizeof(out_phdr));
-    
-    if ( (bytes = RAW_OFFSET - sizeof(out_ehdr) - sizeof(out_phdr)) < 0 )
+
+    if ( num_phdrs > 1 )
+    {
+        endianadjust_phdr32(&note_phdr);
+        do_write(outfd, &note_phdr, sizeof(note_phdr));
+    }
+
+    if ( (bytes = RAW_OFFSET - sizeof(out_ehdr) - sizeof(out_phdr) -
+          ( num_phdrs > 1 ? sizeof(note_phdr) : 0 ) ) < 0 )
     {
         fprintf(stderr, "Header overflow.\n");
         return 1;
@@ -355,9 +447,22 @@ int main(int argc, char **argv)
         endianadjust_shdr32(&out_shdr[i]);
     do_write(outfd, &out_shdr[0], sizeof(out_shdr));
 
-    do_write(outfd, out_shstrtab, sizeof(out_shstrtab));
-    do_write(outfd, buffer, 4-((sizeof(out_shstrtab)+dat_siz)&3));
-
+    if ( num_phdrs > 1 )
+    {
+        endianadjust_shdr32(&out_shdr_extra);
+        /* Append the .note section. */
+        do_write(outfd, &out_shdr_extra, sizeof(out_shdr_extra));
+        /* The normal strings - .text\0.. */
+        do_write(outfd, out_shstrtab, sizeof(out_shstrtab));
+        /* Our .note */
+        do_write(outfd, out_shstrtab_extra, sizeof(out_shstrtab_extra));
+        do_write(outfd, buffer, 4-((sizeof(out_shstrtab)+sizeof(out_shstrtab_extra)+dat_siz)&3));
+    }
+    else
+    {
+        do_write(outfd, out_shstrtab, sizeof(out_shstrtab));
+        do_write(outfd, buffer, 4-((sizeof(out_shstrtab)+dat_siz)&3));
+    }
     close(infd);
     close(outfd);
 
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 961f48f..705fa98 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -31,6 +31,9 @@ OUTPUT_ARCH(i386:x86-64)
 PHDRS
 {
   text PT_LOAD ;
+#if defined(BUILD_ID) && !defined(EFI)
+  note PT_NOTE ;
+#endif
 }
 SECTIONS
 {
@@ -75,6 +78,11 @@ SECTIONS
 
        *(.rodata)
        *(.rodata.*)
+#if defined(BUILD_ID) && defined(EFI)
+       __note_gnu_build_id_start = .;
+       *(.note.gnu.build-id)
+       __note_gnu_build_id_end = .;
+#endif
 
        . = ALIGN(8);
        /* Exception table */
@@ -96,6 +104,21 @@ SECTIONS
        _erodata = .;
   } :text
 
+#if defined(BUILD_ID) && !defined(EFI)
+/*
+ * No mechanism to put an PT_NOTE in the EFI file - so put
+ * it in .data section.
+ */
+  . = ALIGN(4);
+  .note : {
+       __note_gnu_build_id_start = .;
+       *(.note.gnu.build-id)
+       __note_gnu_build_id_end = .;
+       *(.note)
+       *(.note.*)
+  } :note :text
+#endif
+
 #ifdef EFI
   . = ALIGN(MB(2));
 #endif
diff --git a/xen/common/version.c b/xen/common/version.c
index fc9bf42..af87371 100644
--- a/xen/common/version.c
+++ b/xen/common/version.c
@@ -1,5 +1,9 @@
 #include <xen/compile.h>
+#include <xen/errno.h>
+#include <xen/string.h>
+#include <xen/types.h>
 #include <xen/version.h>
+#include <xen/elf.h>
 
 const char *xen_compile_date(void)
 {
@@ -61,6 +65,53 @@ const char *xen_deny(void)
     return "<denied>";
 }
 
+#ifdef BUILD_ID
+#define NT_GNU_BUILD_ID 3
+/* Defined in linker script. */
+extern const Elf_Note __note_gnu_build_id_start[], __note_gnu_build_id_end[];
+
+int xen_build_id(const void **p, ssize_t *len)
+{
+    const Elf_Note *n = __note_gnu_build_id_start;
+    static bool_t checked = 0;
+
+    if ( checked )
+    {
+        *len = n->descsz;
+        *p = ELFNOTE_DESC(n);
+        return 0;
+    }
+    /* --build-id invoked with wrong parameters. */
+    if ( __note_gnu_build_id_end <= __note_gnu_build_id_start )
+        return -ENODATA;
+
+    /* Check for full Note header. */
+    if ( &n[1] > __note_gnu_build_id_end )
+        return -ENODATA;
+
+    /* Check if we really have a build-id. */
+    if ( NT_GNU_BUILD_ID != n->type )
+        return -ENODATA;
+
+    /* Sanity check, name should be "GNU" for ld-generated build-id. */
+    if ( strncmp(ELFNOTE_NAME(n), "GNU", n->namesz) != 0 )
+        return -ENODATA;
+
+    *len = n->descsz;
+    *p = ELFNOTE_DESC(n);
+
+    checked = 1;
+    return 0;
+}
+
+#else
+
+int xen_build_id(const void **p, ssize_t *len)
+{
+    return -ENODATA;
+}
+#endif
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/include/xen/version.h b/xen/include/xen/version.h
index 016a56c..a461c85 100644
--- a/xen/include/xen/version.h
+++ b/xen/include/xen/version.h
@@ -13,4 +13,7 @@ const char *xen_extra_version(void);
 const char *xen_changeset(void);
 const char *xen_banner(void);
 const char *xen_deny(void);
+#include <xen/types.h>
+int xen_build_id(const void **p, ssize_t *len);
+
 #endif /* __XEN_VERSION_H__ */
-- 
2.5.0


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