From ca636ca673e3b7ad5132ab1659598f6dffbcd8f7 Mon Sep 17 00:00:00 2001
From: Matt Turner <mattst88@gmail.com>
Date: Fri, 24 Nov 2017 23:09:01 -0800
Subject: Add build_id_find_nhdr_by_symbol()

---
 Makefile   |  5 +++--
 build-id.c | 40 ++++++++++++++++++++++++++++++++++++++--
 build-id.h |  3 +++
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index 5c4295f..a1a7207 100644
--- a/Makefile
+++ b/Makefile
@@ -19,12 +19,13 @@
 # FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 # IN THE SOFTWARE.
 
+LDLIBS = -ldl
 LDFLAGS = -Wl,--build-id=sha1
 
 all: build-id so-build-id
 
 build-id: test.o build-id.o
-	$(CC) $(LDFLAGS) $^ -o $@
+	$(CC) $(LDFLAGS) $^ -o $@ $(LDLIBS)
 
 so-build-id: so-test.o libbuild-id.so
 	$(CC) $(LDFLAGS) $^ -o $@
@@ -33,7 +34,7 @@ shared-build-id.o: build-id.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) -fPIC -c $^ -o $@
 
 libbuild-id.so: shared-build-id.o
-	$(CC) $(LDFLAGS) -shared $^ -o $@
+	$(CC) $(LDFLAGS) -shared $^ -o $@ $(LDLIBS)
 
 clean:
 	rm -f build-id so-build-id libbuild-id.so *.o
diff --git a/build-id.c b/build-id.c
index 96bbf3e..bd6faae 100644
--- a/build-id.c
+++ b/build-id.c
@@ -23,6 +23,7 @@
  */
 
 #define _GNU_SOURCE
+#include <dlfcn.h>
 #include <link.h>
 #include <stddef.h>
 #include <string.h>
@@ -38,8 +39,16 @@ struct build_id_note {
     uint8_t build_id[0];
 };
 
+enum tag { BY_NAME, BY_SYMBOL };
+
 struct callback_data {
-    const char *name;
+    union {
+        const char *name;
+
+        /* Base address of shared object, taken from Dl_info::dli_fbase */
+        const void *dli_fbase;
+    };
+    enum tag tag;
     struct build_id_note *note;
 };
 
@@ -48,7 +57,10 @@ build_id_find_nhdr_callback(struct dl_phdr_info *info, size_t size, void *data_)
 {
     struct callback_data *data = data_;
 
-    if (strcmp(info->dlpi_name, data->name) != 0)
+    if (data->tag == BY_NAME && strcmp(info->dlpi_name, data->name) != 0)
+        return 0;
+
+    if (data->tag == BY_SYMBOL && (void *)info->dlpi_addr != data->dli_fbase)
         return 0;
 
     for (unsigned i = 0; i < info->dlpi_phnum; i++) {
@@ -83,6 +95,7 @@ const struct build_id_note *
 build_id_find_nhdr_by_name(const char *name)
 {
     struct callback_data data = {
+        .tag = BY_NAME,
         .name = name,
         .note = NULL,
     };
@@ -93,6 +106,29 @@ build_id_find_nhdr_by_name(const char *name)
     return data.note;
 }
 
+const struct build_id_note *
+build_id_find_nhdr_by_symbol(const void *symbol)
+{
+    Dl_info info;
+
+    if (!dladdr(symbol, &info))
+        return NULL;
+
+    if (!info.dli_fbase)
+        return NULL;
+
+    struct callback_data data = {
+        .tag = BY_SYMBOL,
+        .dli_fbase = info.dli_fbase,
+        .note = NULL,
+    };
+
+    if (!dl_iterate_phdr(build_id_find_nhdr_callback, &data))
+        return NULL;
+
+    return data.note;
+}
+
 ElfW(Word)
 build_id_length(const struct build_id_note *note)
 {
diff --git a/build-id.h b/build-id.h
index 8505d9b..e3e64cf 100644
--- a/build-id.h
+++ b/build-id.h
@@ -30,6 +30,9 @@ struct build_id_note;
 const struct build_id_note *
 build_id_find_nhdr_by_name(const char *name);
 
+const struct build_id_note *
+build_id_find_nhdr_by_symbol(const void *symbol);
+
 ElfW(Word)
 build_id_length(const struct build_id_note *note);
 
-- 
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