std: look for build-id section

Author: daurnimator

std/debug.zig

@@ -1023,6 +1023,7 @@ pub fn openElfDebugInfo(
     var debug_line: ?DwarfInfo.Section = null;
     var debug_ranges: ?DwarfInfo.Section = null;
     var gnu_debuglink: ?DwarfInfo.Section = null;
+    var note_gnu_build_id: ?elf.SectionHeader.NoteDescription = null;
     var n: usize = 0; // count number of sections found to exit loop early
 
     var it = efile.sectionIterator();
@@ -1056,12 +1057,19 @@ pub fn openElfDebugInfo(
             };
         } else {
             // don't want to increment `n`
+
+            if (note_gnu_build_id == null) if (try elf_section.isNote(&efile, "GNU", elf.NT_GNU_BUILD_ID)) |note_description| {
+                // is allowed to be in any note section, not just .note.gnu.build-id
+                note_gnu_build_id = note_description;
+            };
+
             if (gnu_debuglink == null and try elf_section.nameIs(&efile, ".gnu_debuglink")) {
                 gnu_debuglink = DwarfInfo.Section{
                     .offset = elf_section.offset,
                     .size = elf_section.size,
                 };
             }
+
             continue;
         }
 
@@ -1085,7 +1093,10 @@ pub fn openElfDebugInfo(
 
     // Look for external debug info
 
-    // TODO: look for note.gnu.build-id. build-id should be preferred over debuglink
+    // build-id is preferred over debuglink
+    if (note_gnu_build_id) |note_description| {
+        // TODO: look in global debug directories for .build-id/nn/nnnnnnnn.debug
+    }
 
     if (gnu_debuglink) |debuglink| {
         if (openGNUDebugLink(allocator, debuglink, elf_seekable_stream, elf_in_stream, efile.endian)) |di| {

std/elf.zig

@@ -267,6 +267,8 @@ pub const SHT_HIPROC = 0x7fffffff;
 pub const SHT_LOUSER = 0x80000000;
 pub const SHT_HIUSER = 0xffffffff;
 
+pub const NT_GNU_BUILD_ID = 3;
+
 pub const STB_LOCAL = 0;
 pub const STB_GLOBAL = 1;
 pub const STB_WEAK = 2;
@@ -362,6 +364,47 @@ pub const SectionHeader = struct {
         try elf.seekable_stream.seekTo(name_offset);
         return try isString(elf, name);
     }
+
+    pub const NoteDescription = struct {
+        elf: *Elf,
+        start_pos: usize,
+        len: usize,
+    };
+    pub fn isNote(elf_section: SectionHeader, elf: *Elf, name: []const u8, wanted_type: u64) !?NoteDescription {
+        if (elf_section.sh_type != SHT_NOTE) return null;
+
+        try elf.seekable_stream.seekTo(elf_section.offset);
+
+        var desc_len: usize = undefined;
+
+        if (elf.is_64) {
+            const hdr = Elf64_Nhdr {
+                .n_namesz = try elf.in_stream.readInt(Elf64_Word, elf.endian),
+                .n_descsz = try elf.in_stream.readInt(Elf64_Word, elf.endian),
+                .n_type = try elf.in_stream.readInt(Elf64_Word, elf.endian),
+            };
+            if (name.len + 1 != hdr.n_namesz) return null;
+            if (wanted_type != hdr.n_type) return null;
+            desc_len = hdr.n_descsz;
+        } else {
+            const hdr = Elf32_Nhdr {
+                .n_namesz = try elf.in_stream.readInt(Elf32_Word, elf.endian),
+                .n_descsz = try elf.in_stream.readInt(Elf32_Word, elf.endian),
+                .n_type = try elf.in_stream.readInt(Elf32_Word, elf.endian),
+            };
+            if (name.len + 1 != hdr.n_namesz) return null;
+            if (wanted_type != hdr.n_type) return null;
+            desc_len = hdr.n_descsz;
+        }
+
+        if (!try isString(elf, name)) return null;
+
+        return NoteDescription {
+            .elf = elf,
+            .start_pos = try elf.seekable_stream.getPos(),
+            .len = desc_len,
+        };
+    }
 };
 
 pub const Elf = struct {