cmd/link: pe .rdata section should not need to be IMAGE_SCN_MEM_EXECUTE #25926
Description
CL 115975 split .text section into .text (code) and .rdata (read only data).
I expected .rdata not to need IMAGE_SCN_MEM_EXECUTE flag set (flag that instructs Windows to allow code execution in that section).
Unfortunately, when I run debug/pe tests, TestInternalLinkerDWARF failed with
--- FAIL: TestInternalLinkerDWARF (0.96s)
file_test.go:348: running test executable failed: exit status 2 Exception 0xc0000005 0x8 0x4d9058 0x4d9058
PC=0x4d9058
runtime: unknown pc 0x4d9058
...
I managed to fix test by adding IMAGE_SCN_MEM_EXECUTE to .rdata section. I did not have time to investigate why IMAGE_SCN_MEM_EXECUTE was required, so I just put TODO for later https://go-review.googlesource.com/c/go/+/115975/3/src/cmd/link/internal/ld/pe.go#1346
I had some time to debug this. TestInternalLinkerDWARF builds a.exe executable. I run a.exe in gdb. And I can see that the crash is happening in x_cgo_thread_start. All other functions do have debug information, but this function does not. The asm for x_cgo_thread_start is:
(gdb) disas
Dump of assembler code for function x_cgo_thread_start:
0x0000000000401330 <+0>: push %rbx
0x0000000000401331 <+1>: sub $0x20,%rsp
0x0000000000401335 <+5>: mov %rcx,%rbx
0x0000000000401338 <+8>: mov $0x18,%ecx
=> 0x000000000040133d <+13>: callq 0x4d9058 <runtime.etypes+24>
0x0000000000401342 <+18>: test %rax,%rax
0x0000000000401345 <+21>: je 0x40136a <x_cgo_thread_start+58>
0x0000000000401347 <+23>: mov (%rbx),%rdx
0x000000000040134a <+26>: mov %rax,%rcx
0x000000000040134d <+29>: mov %rdx,(%rax)
0x0000000000401350 <+32>: mov 0x8(%rbx),%rdx
0x0000000000401354 <+36>: mov %rdx,0x8(%rax)
0x0000000000401358 <+40>: mov 0x10(%rbx),%rdx
0x000000000040135c <+44>: mov %rdx,0x10(%rax)
0x0000000000401360 <+48>: add $0x20,%rsp
0x0000000000401364 <+52>: pop %rbx
0x0000000000401365 <+53>: jmpq 0x401430 <_cgo_sys_thread_start>
0x000000000040136a <+58>: callq *0x149cf0(%rip) # 0x54b060
0x0000000000401370 <+64>: mov $0x2b,%r8d
0x0000000000401376 <+70>: mov $0x1,%edx
0x000000000040137b <+75>: lea 0x60(%rax),%r9
0x000000000040137f <+79>: lea 0xd4fda(%rip),%rcx # 0x4d6360 <runtime/cgo(.rdata)>
0x0000000000401386 <+86>: callq 0x4d9040 <runtime.etypes>
0x000000000040138b <+91>: callq 0x4d9048 <runtime.etypes+8>
0x0000000000401390 <+96>: nop
See how call into _beginthread says <runtime.etypes+24> ? It appears _beginthread function end-up in .rdata section.
(gdb) disas
Dump of assembler code for function runtime.etypes:
0x00000000004d9040 <+0>: jmpq *0x54b040
0x00000000004d9047 <+7>: nop
0x00000000004d9048 <+8>: jmpq *0x54b058
0x00000000004d904f <+15>: nop
0x00000000004d9050 <+16>: jmpq *0x54b050
0x00000000004d9057 <+23>: nop
=> 0x00000000004d9058 <+24>: jmpq *0x54b038
0x00000000004d905f <+31>: nop
0x00000000004d9060 <+32>: jmpq *0x54b048
0x00000000004d9067 <+39>: nop
0x00000000004d9068 <+40>: add %al,(%rax)
0x00000000004d906a <+42>: add %al,(%rax)
0x00000000004d906c <+44>: add %al,(%rax)
0x00000000004d906e <+46>: add %al,(%rax)
0x00000000004d9070 <+48>: add %al,(%rax)
0x00000000004d9072 <+50>: add %al,(%rax)
0x00000000004d9074 <+52>: add %al,(%rax)
0x00000000004d9076 <+54>: add %al,(%rax)
0x00000000004d9078 <+56>: add %al,(%rax)
0x00000000004d907a <+58>: add %al,(%rax)
0x00000000004d907c <+60>: add %al,(%rax)
0x00000000004d907e <+62>: add %al,(%rax)
End of assembler dump.
(gdb) si
Program received signal SIGSEGV, Segmentation fault.
0x00000000004d9058 in runtime.etypes ()
(gdb)
It is no surprise that this code cannot execute - it needs IMAGE_SCN_MEM_EXECUTE.
How is _beginthread code
0x00000000004d9058 <+24>: jmpq *0x54b038
end-up in .rdata section? How can we make it go into .text section?
I am hoping that @ianlancetaylor has some ideas about this. :-)
Alex