mirror of
https://github.com/tinygo-org/tinygo.git
synced 2026-08-03 18:47:47 +00:00
Ayke van Laethem
4ec1e58aa6
We have an optimization for this specific pattern, but it's really just a hack. With the addition of unsafe.Add in Go 1.17 we can directly specify the intent instead and eventually remove this special case. The code is also easier to read.
115 lines
3.5 KiB
Go
115 lines
3.5 KiB
Go
//go:build darwin
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package runtime
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import "unsafe"
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import "C" // dummy import so that os_darwin.c works
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const GOOS = "darwin"
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const (
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// See https://github.com/golang/go/blob/master/src/syscall/zerrors_darwin_amd64.go
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flag_PROT_READ = 0x1
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flag_PROT_WRITE = 0x2
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flag_MAP_PRIVATE = 0x2
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flag_MAP_ANONYMOUS = 0x1000 // MAP_ANON
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)
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// Source: https://opensource.apple.com/source/Libc/Libc-1439.100.3/include/time.h.auto.html
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const (
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clock_REALTIME = 0
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clock_MONOTONIC_RAW = 4
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)
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// https://opensource.apple.com/source/xnu/xnu-7195.141.2/EXTERNAL_HEADERS/mach-o/loader.h.auto.html
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type machHeader struct {
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magic uint32
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cputype uint32
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cpusubtype uint32
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filetype uint32
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ncmds uint32
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sizeofcmds uint32
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flags uint32
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reserved uint32
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}
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// Struct for the LC_SEGMENT_64 load command.
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type segmentLoadCommand struct {
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cmd uint32 // LC_SEGMENT_64
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cmdsize uint32
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segname [16]byte
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vmaddr uintptr
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vmsize uintptr
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fileoff uintptr
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filesize uintptr
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maxprot uint32
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initprot uint32
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nsects uint32
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flags uint32
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}
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// MachO header of the currently running process.
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//
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//go:extern _mh_execute_header
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var libc_mh_execute_header machHeader
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// Mark global variables.
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// The MachO linker doesn't seem to provide symbols for the start and end of the
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// data section. There is get_etext, get_edata, and get_end, but these are
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// undocumented and don't work with ASLR (which is enabled by default).
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// Therefore, read the MachO header directly.
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func markGlobals() {
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// Here is a useful blog post to understand the MachO file format:
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// https://h3adsh0tzz.com/2020/01/macho-file-format/
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const (
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MH_MAGIC_64 = 0xfeedfacf
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LC_SEGMENT_64 = 0x19
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VM_PROT_WRITE = 0x02
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)
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// Sanity check that we're actually looking at a MachO header.
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if gcAsserts && libc_mh_execute_header.magic != MH_MAGIC_64 {
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runtimePanic("gc: unexpected MachO header")
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}
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// Iterate through the load commands.
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// Because we're only interested in LC_SEGMENT_64 load commands, cast the
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// pointer to that struct in advance.
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var offset uintptr
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var hasOffset bool
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cmd := (*segmentLoadCommand)(unsafe.Pointer(uintptr(unsafe.Pointer(&libc_mh_execute_header)) + unsafe.Sizeof(machHeader{})))
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for i := libc_mh_execute_header.ncmds; i != 0; i-- {
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if cmd.cmd == LC_SEGMENT_64 {
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if cmd.fileoff == 0 && cmd.nsects != 0 {
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// Detect ASLR offset by checking fileoff and nsects. This
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// locates the __TEXT segment. This matches getsectiondata:
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// https://opensource.apple.com/source/cctools/cctools-973.0.1/libmacho/getsecbyname.c.auto.html
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offset = uintptr(unsafe.Pointer(&libc_mh_execute_header)) - cmd.vmaddr
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hasOffset = true
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}
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if cmd.maxprot&VM_PROT_WRITE != 0 {
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// Found a writable segment, which may contain Go globals.
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if gcAsserts && !hasOffset {
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// No ASLR offset detected. Did the __TEXT segment come
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// after the __DATA segment?
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// Note that when ASLR is disabled (for example, when
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// running inside lldb), the offset is zero. That's why we
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// need a separate hasOffset for this assert.
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runtimePanic("gc: did not detect ASLR offset")
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}
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// Scan this segment for GC roots.
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// This could be improved by only reading the memory areas
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// covered by sections. That would reduce the amount of memory
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// scanned a little bit (up to a single VM page).
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markRoots(offset+cmd.vmaddr, offset+cmd.vmaddr+cmd.vmsize)
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}
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}
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// Move on to the next load command (wich may or may not be a
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// LC_SEGMENT_64).
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cmd = (*segmentLoadCommand)(unsafe.Add(unsafe.Pointer(cmd), cmd.cmdsize))
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}
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}
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