On Mon, Sep 14, 2020 at 06:37:33PM +0200, Thomas Huth wrote:
> On 14/09/2020 16.45, Roman Bolshakov wrote:
> > The difference is only realmode test which doesn't work if built by
> > clang.
>
> Hmm, if you got some spare minutes, could you check if it works when
> replacing the asm() statements there with asm volatile() ?
> (Otherwise I'll check it if I got some spare time again ... so likely
> not this week ;-))
>
Hi Thomas,
Sure. There are only two places where volatile is missed (inside functions) and
would make sense to add it. Unfortunately it doesn't help much:
diff --git a/x86/realmode.c b/x86/realmode.c
index 7c2d776..30691bc 100644
--- a/x86/realmode.c
+++ b/x86/realmode.c
@@ -271,7 +271,7 @@ static void report(const char *name, u16 regs_ignore, _Bool ok)
}
#define MK_INSN(name, str) \
- asm ( \
+ asm volatile ( \
".pushsection .data.insn \n\t" \
"insn_" #name ": \n\t" \
".word 1001f, 1002f - 1001f \n\t" \
@@ -1448,7 +1448,7 @@ static void test_cpuid(void)
eax = inregs.eax;
ecx = inregs.ecx;
- asm("cpuid" : "+a"(eax), "=b"(ebx), "+c"(ecx), "=d"(edx));
+ asm volatile("cpuid" : "+a"(eax), "=b"(ebx), "+c"(ecx), "=d"(edx));
exec_in_big_real_mode(&insn_cpuid);
report("cpuid", R_AX|R_BX|R_CX|R_DX,
outregs.eax == eax && outregs.ebx == ebx
So, I looked further and noticed that multiboot header is missed in the flat
binary:
".section .init \n\t"
".code32 \n\t"
"mb_magic = 0x1BADB002 \n\t"
"mb_flags = 0x0 \n\t"
"# multiboot header \n\t"
".long mb_magic, mb_flags, 0 - (mb_magic + mb_flags) \n\t"
But I can see it in the object file. So, I believe linker didn't place .init
section at 0x1000 despite realmode.lds instruction:
SECTIONS
{
. = 16K;
stext = .;
.text : { *(.init) *(.text) }
. = ALIGN(4K);
.data : { *(.data) *(.rodata*) }
. = ALIGN(16);
.bss : { *(.bss) }
edata = .;
}
ENTRY(start)
If I read it correctly, given the segment in realmode.elf:
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x001000 0x00004000 0x00004000 0x03c90 0x03c90 R E 0x1000
Here's multiboot header in GCC-compiled binary:
00001000: 02b0ad1b 00000000 fe4f52e4 66b83412 .........OR.f.4.
Here's the same location in clang-compiled binary:
00001000: 04000000 14000000 03000000 474e5500 ............GNU.
Here's verbose invocation of linker by clang:
$ clang-10 -v -m32 -nostdlib -o x86/realmode.elf -Wl,-m,elf_i386 -Wl,-T,/home/roolebo
/dev/kvm-unit-tests/x86/realmode.lds x86/realmode.o
clang version 10.0.0-4ubuntu1
Target: i386-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
"/usr/bin/ld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o x86/realmode.elf -L/lib/../lib32 -L/usr/lib/../lib32 -L/usr/lib/llvm-10/bin/../lib -L/lib -L/usr/lib -m elf_i386 -T /home/roolebo/dev/kvm-unit-tests/x86/realmode.lds x86/realmode.o
(BTW, I'm surprised to see -dynamic-linker being set, it has little sense for
the tests but that's likely because -static is not passed explicitly)
And if I pass --print-map option to the linker I can see that GNU build id is
placed at 0x4000:
$ clang-10 -m32 -nostdlib -o x86/realmode.elf -Wl,-M -Wl,-m,elf_i386 -Wl,-T,/home/roo
lebo/dev/kvm-unit-tests/x86/realmode.lds x86/realmode.o
Discarded input sections
.llvm_addrsig 0x0000000000000000 0x122 x86/realmode.o
Memory Configuration
Name Origin Length Attributes
*default* 0x0000000000000000 0xffffffffffffffff
Linker script and memory map
0x0000000000004000 . = 0x4000
0x0000000000004000 stext = .
.note.gnu.build-id
0x0000000000004000 0x24
.note.gnu.build-id
0x0000000000004000 0x24 x86/realmode.o
.text 0x0000000000004030 0x290c
*(.init)
.init 0x0000000000004030 0xc x86/realmode.o
*(.text)
*fill* 0x000000000000403c 0x4
.text 0x0000000000004040 0x28fc x86/realmode.o
0x000000000000404c start
0x00000000000040a0 realmode_start
.text.insn 0x000000000000693c 0x40e
.text.insn 0x000000000000693c 0x40e x86/realmode.o
.iplt 0x0000000000006d4a 0x0
.iplt 0x0000000000006d4a 0x0 x86/realmode.o
.rel.dyn 0x0000000000006d4c 0x0
.rel.got 0x0000000000006d4c 0x0 x86/realmode.o
.rel.iplt 0x0000000000006d4c 0x0 x86/realmode.o
0x0000000000007000 . = ALIGN (0x1000)
.data 0x0000000000007000 0x2538
*(.data)
.data 0x0000000000007000 0x1064 x86/realmode.o
0x0000000000008000 r_gdt
0x0000000000008018 r_gdt_descr
0x000000000000801e r_idt_descr
*(.rodata*)
.rodata.str1.1
0x0000000000008064 0x4d3 x86/realmode.o
0x4f9 (size before relaxing)
*fill* 0x0000000000008537 0x1
.rodata 0x0000000000008538 0x1000 x86/realmode.o
.data.insn 0x0000000000009538 0x21c
.data.insn 0x0000000000009538 0x21c x86/realmode.o
.got 0x0000000000009754 0x0
.got 0x0000000000009754 0x0 x86/realmode.o
.got.plt 0x0000000000009754 0x0
.got.plt 0x0000000000009754 0x0 x86/realmode.o
.igot.plt 0x0000000000009754 0x0
.igot.plt 0x0000000000009754 0x0 x86/realmode.o
0x0000000000009760 . = ALIGN (0x10)
.bss 0x0000000000009760 0x284
*(.bss)
.bss 0x0000000000009760 0x284 x86/realmode.o
0x0000000000009764 tmp_stack
0x00000000000099e4 edata = .
LOAD x86/realmode.o
OUTPUT(x86/realmode.elf elf32-i386)
.debug_str 0x0000000000000000 0x509
.debug_str 0x0000000000000000 0x509 x86/realmode.o
0x59f (size before relaxing)
.debug_loc 0x0000000000000000 0x484
.debug_loc 0x0000000000000000 0x484 x86/realmode.o
.debug_abbrev 0x0000000000000000 0x212
.debug_abbrev 0x0000000000000000 0x212 x86/realmode.o
.debug_info 0x0000000000000000 0x1935
.debug_info 0x0000000000000000 0x1935 x86/realmode.o
.comment 0x0000000000000000 0x1f
.comment 0x0000000000000000 0x1f x86/realmode.o
0x20 (size before relaxing)
.note.GNU-stack
0x0000000000000000 0x0
.note.GNU-stack
0x0000000000000000 0x0 x86/realmode.o
.debug_frame 0x0000000000000000 0x824
.debug_frame 0x0000000000000000 0x824 x86/realmode.o
.debug_line 0x0000000000000000 0xc06
.debug_line 0x0000000000000000 0xc06 x86/realmode.o
So, a workaround for that could be adding '-Wl,--build-id=none' to the
makefile rule for realmode.elf. Then multiboot magic is placed properly
at 0x4000 instead of 0x4030. Unfortunately it doesn't help with the
test :-)
-Roman
