golang/go

cmd/objdump: doesn't disassemble the last function in a macho file

Open

#78811 opened on Apr 16, 2026

View on GitHub
 (4 comments) (1 reaction) (1 assignee)Go (133,883 stars) (19,008 forks)batch import
BugReportNeedsFixOS-Darwincompiler/runtimehelp wanted

Description

On darwin, do:

main.c:

void f();
void g();

int main() {
  f();
  g();
  return 0;
}

main.s:

	.globl _f
_f:
	add	x0, x0, x0
	add	x1, x1, x1
	ret

	.globl _g
_g:
	add	x2, x2, x2
	add	x3, x3, x3
	ret

gcc -O3 main.c main.s
go tool objdump a.out

This will print the body of main and f, but does not print the body of g. We get only the TEXT line:

TEXT _main(SB) 
  :0			0x100000328		a9bf7bfd		STP.W (R29, R30), -16(RSP)	
  :0			0x10000032c		910003fd		MOVD RSP, R29			
  :0			0x100000330		94000005		CALL _f(SB)			
  :0			0x100000334		94000007		CALL 7(PC)			
  :0			0x100000338		52800000		MOVW $0, R0			
  :0			0x10000033c		a8c17bfd		LDP.P 16(RSP), (R29, R30)	
  :0			0x100000340		d65f03c0		RET				

TEXT _f(SB) 
  :0			0x100000344		8b000000		ADD R0, R0, R0		
  :0			0x100000348		8b010021		ADD R1, R1, R1		
  :0			0x10000034c		d65f03c0		RET			

TEXT _g(SB)

Looks like the size of the g symbol is set to 0. The code that does this, in cmd/internal/objfile/macho.go:symbols(), sets the size of a symbol by looking at the address of the next higher symbol and subtracting. For the last symbol in the file, this doesn't work. We should probably use the end of the containing section or something.

The "CALL 7(PC)" instead of "CALL _g(SB)" might be related.

Contributor guide

Tech stack
go
Domain
clitooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Go programmingMach O file formatobjdump usage
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the `symbols()` function in `cmd/internal/objfile/macho.go`. The current implementation calculates symbol size by subtracting the next symbol's address, which fails for the last symbol. Consider using the end of the containing section (e.g., `sect.End`) to compute the size. Test with the provided example assembly file using `gcc O3 main.c main.s` and `go tool objdump a.out`.