macho_update_uuid.go

     1  // Copyright 2024 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package ld
     6  
     7  // This file provides helper functions for updating/rewriting the UUID
     8  // load command within a Go go binary generated on Darwin using
     9  // external linking. Why is it necessary to update the UUID load
    10  // command? See issue #64947 for more detail, but the short answer is
    11  // that newer versions of the Macos toolchain (the newer linker in
    12  // particular) appear to compute the UUID based not just on the
    13  // content of the object files being linked but also on things like
    14  // the timestamps/paths of the objects; this makes it
    15  // difficult/impossible to support reproducible builds. Since we try
    16  // hard to maintain build reproducibility for Go, the APIs here
    17  // compute a new UUID (based on the Go build ID) and write it to the
    18  // final executable generated by the external linker.
    19  
    20  import (
    21  	"cmd/internal/hash"
    22  	imacho "cmd/internal/macho"
    23  
    24  	"debug/macho"
    25  	"io"
    26  	"os"
    27  )
    28  
    29  // uuidFromGoBuildId hashes the Go build ID and returns a slice of 16
    30  // bytes suitable for use as the payload in a Macho LC_UUID load
    31  // command.
    32  func uuidFromGoBuildId(buildID string) []byte {
    33  	if buildID == "" {
    34  		return make([]byte, 16)
    35  	}
    36  	hashedBuildID := hash.Sum32([]byte(buildID))
    37  	rv := hashedBuildID[:16]
    38  
    39  	// RFC 4122 conformance (see RFC 4122 Sections 4.2.2, 4.1.3). We
    40  	// want the "version" of this UUID to appear as 'hashed' as opposed
    41  	// to random or time-based.  This is something of a fiction since
    42  	// we're not actually hashing using MD5 or SHA1, but it seems better
    43  	// to use this UUID flavor than any of the others. This is similar
    44  	// to how other linkers handle this (for example this code in lld:
    45  	// https://github.com/llvm/llvm-project/blob/2a3a79ce4c2149d7787d56f9841b66cacc9061d0/lld/MachO/Writer.cpp#L524).
    46  	rv[6] &= 0x0f
    47  	rv[6] |= 0x30
    48  	rv[8] &= 0x3f
    49  	rv[8] |= 0xc0
    50  
    51  	return rv
    52  }
    53  
    54  // machoRewriteUuid copies over the contents of the Macho executable
    55  // exef into the output file outexe, and in the process updates the
    56  // LC_UUID command to a new value recomputed from the Go build id.
    57  func machoRewriteUuid(ctxt *Link, exef *os.File, exem *macho.File, outexe string) error {
    58  	outf, err := os.OpenFile(outexe, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0755)
    59  	if err != nil {
    60  		return err
    61  	}
    62  	defer outf.Close()
    63  
    64  	// Copy over the file.
    65  	if _, err := io.Copy(outf, exef); err != nil {
    66  		return err
    67  	}
    68  
    69  	// Locate the portion of the binary containing the load commands.
    70  	cmdOffset := imacho.FileHeaderSize(exem)
    71  	if _, err := outf.Seek(cmdOffset, 0); err != nil {
    72  		return err
    73  	}
    74  
    75  	// Read the load commands, looking for the LC_UUID cmd. If/when we
    76  	// locate it, overwrite it with a new value produced by
    77  	// uuidFromGoBuildId.
    78  	reader := imacho.NewLoadCmdUpdater(outf, exem.ByteOrder, cmdOffset)
    79  	for i := uint32(0); i < exem.Ncmd; i++ {
    80  		cmd, err := reader.Next()
    81  		if err != nil {
    82  			return err
    83  		}
    84  		if cmd.Cmd == imacho.LC_UUID {
    85  			var u uuidCmd
    86  			if err := reader.ReadAt(0, &u); err != nil {
    87  				return err
    88  			}
    89  			clear(u.Uuid[:])
    90  			copy(u.Uuid[:], buildinfo)
    91  			if err := reader.WriteAt(0, &u); err != nil {
    92  				return err
    93  			}
    94  			break
    95  		}
    96  	}
    97  
    98  	// We're done
    99  	return nil
   100  }
   101
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