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Re: Does your .eh_frame_hdr change work with DW_EH_PE_absptr?


On Thu, Feb 14, 2002 at 09:36:28PM +0100, Jakub Jelinek wrote:
> On Thu, Feb 14, 2002 at 12:20:12PM -0800, H . J . Lu wrote:
> > I don't know what the best way to fix it is. Given the problem I see
> > on Linux/mips, I don't know how well your scheme will work on other
> > platforms. The key is you have to make sure the .eh_frame_hdr section
> > needs no run-time relocation or provide the relocation records for it. 
> > I believe you should require the .eh_frame_hdr section needs no
> > run-time relocation.
> 
> .eh_frame_hdr never needs any run-time relocation.
> It always uses pcrel encoding.
> 

Here is the problem:

#ifndef ASM_PREFERRED_EH_DATA_FORMAT
#define ASM_PREFERRED_EH_DATA_FORMAT(CODE,GLOBAL)  DW_EH_PE_absptr
#endif

It is true that .eh_frame_hdr itself doesn't need run-time relocation.
But the VALUEs stored in .eh_frame_hdr may be the one before run-time
relocation. It is the case for Linux/mips, which uses DW_EH_PE_absptr.
You have to make sure the VALUEs stored in .eh_frame_hdr need no
run-time relocations. But there are no checks in elf-eh-frame.c for
run-time relocations on .eh_frame_hdr. As the result, ld generates the
wrong binary silently. I am enclosing a simple asm code. The initial
loc field in FDE is

$LFB1:
	...
	.4byte	$LFB1

It needs the run-time relocation. But the value stored in .eh_frame_hdr
is before the run-time relocation is made.

BTW, this gcc patch seems to pass my very limited test.


H.J.
---
2002-02-14  H.J. Lu <hjl@gnu.org>

	* config/mips/linux.h (ASM_PREFERRED_EH_DATA_FORMAT): New.

--- gcc/config/mips/linux.h.eh	Fri Feb  8 17:33:04 2002
+++ gcc/config/mips/linux.h	Thu Feb 14 12:25:22 2002
@@ -283,6 +283,9 @@ void FN ()							\
    pseudo-ops.  */
 #define FUNCTION_NAME_ALREADY_DECLARED
 
+#define ASM_PREFERRED_EH_DATA_FORMAT(CODE,GLOBAL)       \
+  (((GLOBAL) ? DW_EH_PE_indirect : 0) | DW_EH_PE_pcrel | DW_EH_PE_sdata4)
+
 /* The glibc _mcount stub will save $v0 for us.  Don't mess with saving
    it, since ASM_OUTPUT_REG_PUSH/ASM_OUTPUT_REG_POP do not work in the
    presence of $gp-relative calls.  */
---
	.file	1 "foo.cc"
	.section .mdebug.abi32
	.previous
	.abicalls
	.globl	_Unwind_Resume
	.text
	.align	2
	.globl	main
	.ent	main
		.type		 main,@function
main:
$LFB1:
	.frame	$sp,40,$31		# vars= 0, regs= 3/0, args= 16, extra= 8
	.mask	0x90010000,-8
	.fmask	0x00000000,0
	.set	noreorder
	.cpload	$25
	.set	reorder
	subu	$sp,$sp,40
	.cprestore 16
$LCFI0:
	li	$4,4			# 0x4
	sw	$31,32($sp)
$LCFI1:
	sw	$28,28($sp)
$LCFI2:
	sw	$16,24($sp)
$LCFI3:
	la	$25,__cxa_allocate_exception
	jal	$31,$25
	li	$3,99			# 0x63
	sw	$3,0($2)
	move	$4,$2
	la	$5,_ZTIi
	move	$6,$0
$LEHB0:
	la	$25,__cxa_throw
	jal	$31,$25
$LEHE0:
$L11:
	lw	$28,16($sp)
	li	$2,1			# 0x1
	bne	$5,$2,$L12
	la	$25,__cxa_begin_catch
	jal	$31,$25
	lw	$16,0($2)
	la	$25,__cxa_end_catch
	jal	$31,$25
	move	$2,$16
	lw	$31,32($sp)
	lw	$16,24($sp)
	#nop
	.set	noreorder
	.set	nomacro
	j	$31
	addu	$sp,$sp,40
	.set	macro
	.set	reorder

$L12:
$LEHB1:
	la	$25,_Unwind_Resume
	jal	$31,$25
$LEHE1:
$LFE1:
	.end	main
	.section	.gcc_except_table,"aw",@progbits
	.align	2
$LLSDA1:
	.byte	0xff
	.byte	0x0
	.uleb128 $LLSDATT1-$LLSDATTD1
$LLSDATTD1:
	.byte	0x1
	.uleb128 $LLSDACSE1-$LLSDACSB1
$LLSDACSB1:
	.uleb128 $LEHB0-$LFB1
	.uleb128 $LEHE0-$LEHB0
	.uleb128 $L11-$LFB1
	.uleb128 0x1
	.uleb128 $LEHB1-$LFB1
	.uleb128 $LEHE1-$LEHB1
	.uleb128 0x0
	.uleb128 0x0
$LLSDACSE1:
	.byte	0x1
	.byte	0x0
	.align	2
	.word	_ZTIi
$LLSDATT1:
	.text
	.section	.eh_frame,"aw",@progbits
$Lframe1:
	.4byte	$LECIE1-$LSCIE1
$LSCIE1:
	.4byte	0x0
	.byte	0x1
	.ascii	"zPL\000"
	.uleb128 0x1
	.sleb128 4
	.byte	0x40
	.uleb128 0x6
	.byte	0x0
	.4byte	__gxx_personality_v0
	.byte	0x0
	.byte	0xc
	.uleb128 0x1d
	.uleb128 0x0
	.align	2
$LECIE1:
$LSFDE1:
	.4byte	$LEFDE1-$LASFDE1
$LASFDE1:
	.4byte	$LASFDE1-$Lframe1
	.4byte	$LFB1
	.4byte	$LFE1-$LFB1
	.uleb128 0x4
	.4byte	$LLSDA1
	.byte	0x4
	.4byte	$LCFI0-$LFB1
	.byte	0xe
	.uleb128 0x28
	.byte	0x4
	.4byte	$LCFI3-$LCFI0
	.byte	0x11
	.uleb128 0x10
	.sleb128 -4
	.byte	0x11
	.uleb128 0x1c
	.sleb128 -3
	.byte	0x11
	.uleb128 0x40
	.sleb128 -2
	.align	2
$LEFDE1:
	.align	0
	.ident	"GCC: (GNU) 3.1 20020207 (experimental)"


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