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macho: prefill any space between __DATA and __LINKEDIT with 0s if required

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If there are zerofill sections, the loader may copy the contents of
the physical space in file directly into memory and attach that to
the zerofill section. This is a performance optimisation in the loader
but requires us, the linker, to properly zero-out any space between
__DATA and __LINKEDIT segments in file. This is of course completely
skipped if there are no zerofill sections present.
This commit is contained in:
Jakub Konka 2022-09-09 00:01:20 +02:00
parent 0ae2ea671b
commit 8378cde743
1 changed files with 26 additions and 2 deletions
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28
src/link/MachO.zig
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@ -1157,6 +1157,28 @@ fn linkOneShot(self: *MachO, comp: *Compilation, prog_node: *std.Progress.Node)
var ncmds: u32 = 0;
try self.writeLinkeditSegmentData(&ncmds, lc_writer);
// If the last section of __DATA segment is zerofill section, we need to ensure
// that the free space between the end of the last non-zerofill section of __DATA
// segment and the beginning of __LINKEDIT segment is zerofilled as the loader will
// copy-paste this space into memory for quicker zerofill operation.
if (self.data_segment_cmd_index) |data_seg_id| blk: {
var physical_zerofill_start: u64 = 0;
const section_indexes = self.getSectionIndexes(data_seg_id);
for (self.sections.items(.header)[section_indexes.start..section_indexes.end]) |header| {
if (header.isZerofill() and header.size > 0) break;
physical_zerofill_start = header.offset + header.size;
} else break :blk;
const linkedit = self.segments.items[self.linkedit_segment_cmd_index.?];
const physical_zerofill_size = linkedit.fileoff - physical_zerofill_start;
if (physical_zerofill_size > 0) {
var padding = try self.base.allocator.alloc(u8, physical_zerofill_size);
defer self.base.allocator.free(padding);
mem.set(u8, padding, 0);
try self.base.file.?.pwriteAll(padding, physical_zerofill_start);
}
}
try writeDylinkerLC(&ncmds, lc_writer);
try self.writeMainLC(&ncmds, lc_writer);
try self.writeDylibIdLC(&ncmds, lc_writer);
@ -5690,8 +5712,10 @@ fn writeHeader(self: *MachO, ncmds: u32, sizeofcmds: u32) !void {
else => unreachable,
}
if (self.getSectionByName("__DATA", "__thread_vars")) |_| {
header.flags |= macho.MH_HAS_TLV_DESCRIPTORS;
if (self.getSectionByName("__DATA", "__thread_vars")) |sect_id| {
if (self.sections.items(.header)[sect_id].size > 0) {
header.flags |= macho.MH_HAS_TLV_DESCRIPTORS;
}
}
header.ncmds = ncmds;