[PATCH 1/2] xen/build-id: Introduce is_gnu_build_id()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Broken out of the subsequent change for legibility reasons.  It will be used
to replace xen_build_id_check().

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/common/version.c         | 33 +++++++++++++++++++++++++++++++++
 xen/include/xen/elfstructs.h |  2 ++
 xen/include/xen/version.h    |  2 ++
 3 files changed, 37 insertions(+)

diff --git a/xen/common/version.c b/xen/common/version.c
index 80e7a9b981cf..7eb8a0221d82 100644
--- a/xen/common/version.c
+++ b/xen/common/version.c
@@ -114,6 +114,37 @@ void print_version(void)
 /* Defined in linker script. */
 extern const Elf_Note __note_gnu_build_id_start[], __note_gnu_build_id_end[];
 
+/*
+ * Check whether @n is a GNU Build ID note.
+ *
+ * We check that:
+ *  - type is NT_GNU_BUILD_ID
+ *  - name is GNU\0
+ *  - desc is non-empty and exactly covering the rest of the buffer
+ *
+ * If this returns true, the contents starts at n + NT_GNU_HDR_SIZE.
+ */
+bool is_gnu_build_id(const Elf_Note *n, unsigned int len)
+{
+    const char *name;
+
+    if ( len <= NT_GNU_HDR_SIZE )
+        return false;
+
+    if ( n->type != NT_GNU_BUILD_ID ||
+         n->namesz != 4 )
+        return false;
+
+    name = ELFNOTE_NAME(n);
+    if ( strncmp(name, "GNU", 4) != 0 )
+        return false;
+
+    if ( len != n->descsz + NT_GNU_HDR_SIZE )
+        return false;
+
+    return true;
+}
+
 int xen_build_id_check(const Elf_Note *n, unsigned int n_sz,
                        const void **p, unsigned int *len)
 {
@@ -234,6 +265,8 @@ static void __init __maybe_unused build_assertions(void)
 {
     BUILD_BUG_ON(sizeof(unsigned long) != sizeof(void (*)(void)));
     BUILD_BUG_ON(sizeof(void *) != sizeof(void (*)(void)));
+
+    BUILD_BUG_ON(NT_GNU_HDR_SIZE != sizeof(Elf_Note) + 4);
 }
 
 /*
diff --git a/xen/include/xen/elfstructs.h b/xen/include/xen/elfstructs.h
index 8ee0d5aa5b15..1f0287aeed49 100644
--- a/xen/include/xen/elfstructs.h
+++ b/xen/include/xen/elfstructs.h
@@ -46,6 +46,8 @@ typedef uint32_t      Elf64_Word;
 typedef int64_t                Elf64_Sxword;
 typedef uint64_t       Elf64_Xword;
 
+#define NT_GNU_HDR_SIZE 16 /* sizeof(Elf_note) + "GNU\0" */
+
 /* Unique build id string format when using --build-id. */
 #define NT_GNU_BUILD_ID 3
 
diff --git a/xen/include/xen/version.h b/xen/include/xen/version.h
index b9fa8d70520d..ee857616f9e4 100644
--- a/xen/include/xen/version.h
+++ b/xen/include/xen/version.h
@@ -2,6 +2,7 @@
 #define __XEN_VERSION_H__
 
 #include <xen/elfstructs.h>
+#include <xen/stdbool.h>
 
 const char *xen_compile_date(void);
 const char *xen_compile_time(void);
@@ -26,6 +27,7 @@ extern unsigned int xen_build_id_len; /* 0 -> No build id. */
 void xen_build_init(void);
 int xen_build_id_check(const Elf_Note *n, unsigned int n_sz,
                        const void **p, unsigned int *len);
+bool is_gnu_build_id(const Elf_Note *n, unsigned int len);
 #else
 static inline void xen_build_init(void) {};
 #endif

base-commit: 2043d22b7e926f508cc23391b4c75325fa7211e9
-- 
2.39.5