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macho: do not preempt segment headers; do it when commiting to file

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This way, tracking segment-to-section mapping becomes a lot easier
since it's effectively just start index plus number of sections
defined within the segment. If a section becomes empty however
care needs to be taken to remove the header upon committing to the
final binary.
This commit is contained in:
Jakub Konka 2022-08-02 22:15:07 +02:00
parent 421d3e8d28
commit 90e3268270
3 changed files with 35 additions and 13 deletions
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23
src/link/MachO.zig
View File

@ -4888,13 +4888,26 @@ fn getSegmentAllocBase(self: MachO, indices: []const ?u8) struct { vmaddr: u64,
fn writeSegmentHeaders(self: *MachO, ncmds: *u32, writer: anytype) !void {
for (self.segments.items) |seg, i| {
if (seg.nsects == 0 and
(mem.eql(u8, seg.segName(), "__DATA_CONST") or
mem.eql(u8, seg.segName(), "__DATA"))) continue;
try writer.writeStruct(seg);
const indexes = self.getSectionIndexes(@intCast(u8, i));
var out_seg = seg;
out_seg.cmdsize = @sizeOf(macho.segment_command_64);
out_seg.nsects = 0;
// Update section headers count; any section with size of 0 is excluded
// since it doesn't have any data in the final binary file.
for (self.sections.items(.header)[indexes.start..indexes.end]) |header| {
if (header.size == 0) continue;
out_seg.cmdsize += @sizeOf(macho.section_64);
out_seg.nsects += 1;
}
if (out_seg.nsects == 0 and
(mem.eql(u8, out_seg.segName(), "__DATA_CONST") or
mem.eql(u8, out_seg.segName(), "__DATA"))) continue;
try writer.writeStruct(out_seg);
for (self.sections.items(.header)[indexes.start..indexes.end]) |header| {
if (header.size == 0) continue;
try writer.writeStruct(header);
}

22
src/link/MachO/DebugSymbols.zig
View File

@ -367,16 +367,28 @@ fn writeSegmentHeaders(self: *DebugSymbols, ncmds: *u32, writer: anytype) !void
// Write segment/section headers from the binary file first.
const end = self.base.linkedit_segment_cmd_index.?;
for (self.base.segments.items[0..end]) |seg, i| {
if (seg.nsects == 0 and
(mem.eql(u8, seg.segName(), "__DATA_CONST") or
mem.eql(u8, seg.segName(), "__DATA"))) continue;
const indexes = self.base.getSectionIndexes(@intCast(u8, i));
var out_seg = seg;
out_seg.fileoff = 0;
out_seg.filesize = 0;
try writer.writeStruct(out_seg);
out_seg.cmdsize = @sizeOf(macho.segment_command_64);
out_seg.nsects = 0;
const indexes = self.base.getSectionIndexes(@intCast(u8, i));
// Update section headers count; any section with size of 0 is excluded
// since it doesn't have any data in the final binary file.
for (self.base.sections.items(.header)[indexes.start..indexes.end]) |header| {
if (header.size == 0) continue;
out_seg.cmdsize += @sizeOf(macho.section_64);
out_seg.nsects += 1;
}
if (out_seg.nsects == 0 and
(mem.eql(u8, out_seg.segName(), "__DATA_CONST") or
mem.eql(u8, out_seg.segName(), "__DATA"))) continue;
try writer.writeStruct(out_seg);
for (self.base.sections.items(.header)[indexes.start..indexes.end]) |header| {
if (header.size == 0) continue;
var out_header = header;
out_header.offset = 0;
try writer.writeStruct(out_header);

3
src/link/MachO/dead_strip.zig
View File

@ -43,9 +43,6 @@ fn removeAtomFromSection(atom: *Atom, match: u8, macho_file: *MachO) void {
// The section will be GCed in the next step.
section.last_atom = null;
section.header.size = 0;
const segment = &macho_file.segments.items[section.segment_index];
segment.cmdsize -= @sizeOf(macho.section_64);
segment.nsects -= 1;
}
}