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x86/entry/vdso32: Remove open-coded DWARF in sigreturn.S

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The vdso32 sigreturn.S contains open-coded DWARF bytecode, which
includes a hack for gdb to not try to step back to a previous call
instruction when backtracing from a signal handler.

Neither of those are necessary anymore: the backtracing issue is
handled by ".cfi_entry simple" and ".cfi_signal_frame", both of which
have been supported for a very long time now, which allows the
remaining frame to be built using regular .cfi annotations.

Add a few more register offsets to the signal frame just for good
measure.

Replace the nop on fallthrough of the system call (which should never,
ever happen) with a ud2a trap.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251216212606.1325678-7-hpa@zytor.com
This commit is contained in:
H. Peter Anvin 2025-12-16 13:26:00 -08:00 • committed by Dave Hansen
parent 98d3e99651
commit 884961618e
3 changed files with 39 additions and 114 deletions
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146
arch/x86/entry/vdso/vdso32/sigreturn.S
View file

@ -1,136 +1,54 @@
/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/linkage.h>
#include <asm/unistd_32.h>
#include <asm/dwarf2.h>
#include <asm/asm-offsets.h>
.macro STARTPROC_SIGNAL_FRAME sc
CFI_STARTPROC simple
CFI_SIGNAL_FRAME
/* -4 as pretcode has already been popped */
CFI_DEF_CFA esp, \sc - 4
CFI_OFFSET eip, IA32_SIGCONTEXT_ip
CFI_OFFSET eax, IA32_SIGCONTEXT_ax
CFI_OFFSET ebx, IA32_SIGCONTEXT_bx
CFI_OFFSET ecx, IA32_SIGCONTEXT_cx
CFI_OFFSET edx, IA32_SIGCONTEXT_dx
CFI_OFFSET esp, IA32_SIGCONTEXT_sp
CFI_OFFSET ebp, IA32_SIGCONTEXT_bp
CFI_OFFSET esi, IA32_SIGCONTEXT_si
CFI_OFFSET edi, IA32_SIGCONTEXT_di
CFI_OFFSET es, IA32_SIGCONTEXT_es
CFI_OFFSET cs, IA32_SIGCONTEXT_cs
CFI_OFFSET ss, IA32_SIGCONTEXT_ss
CFI_OFFSET ds, IA32_SIGCONTEXT_ds
CFI_OFFSET eflags, IA32_SIGCONTEXT_flags
.endm
.text
.globl __kernel_sigreturn
.type __kernel_sigreturn,@function
nop /* this guy is needed for .LSTARTFDEDLSI1 below (watch for HACK) */
ALIGN
__kernel_sigreturn:
.LSTART_sigreturn:
popl %eax /* XXX does this mean it needs unwind info? */
STARTPROC_SIGNAL_FRAME IA32_SIGFRAME_sigcontext
popl %eax
CFI_ADJUST_CFA_OFFSET -4
movl $__NR_sigreturn, %eax
int $0x80
.LEND_sigreturn:
SYM_INNER_LABEL(vdso32_sigreturn_landing_pad, SYM_L_GLOBAL)
nop
.size __kernel_sigreturn,.-.LSTART_sigreturn
ud2a
CFI_ENDPROC
.size __kernel_sigreturn,.-__kernel_sigreturn
.globl __kernel_rt_sigreturn
.type __kernel_rt_sigreturn,@function
ALIGN
__kernel_rt_sigreturn:
.LSTART_rt_sigreturn:
STARTPROC_SIGNAL_FRAME IA32_RT_SIGFRAME_sigcontext
movl $__NR_rt_sigreturn, %eax
int $0x80
.LEND_rt_sigreturn:
SYM_INNER_LABEL(vdso32_rt_sigreturn_landing_pad, SYM_L_GLOBAL)
nop
.size __kernel_rt_sigreturn,.-.LSTART_rt_sigreturn
.previous
.section .eh_frame,"a",@progbits
.LSTARTFRAMEDLSI1:
.long .LENDCIEDLSI1-.LSTARTCIEDLSI1
.LSTARTCIEDLSI1:
.long 0 /* CIE ID */
.byte 1 /* Version number */
.string "zRS" /* NUL-terminated augmentation string */
.uleb128 1 /* Code alignment factor */
.sleb128 -4 /* Data alignment factor */
.byte 8 /* Return address register column */
.uleb128 1 /* Augmentation value length */
.byte 0x1b /* DW_EH_PE_pcrel|DW_EH_PE_sdata4. */
.byte 0 /* DW_CFA_nop */
.align 4
.LENDCIEDLSI1:
.long .LENDFDEDLSI1-.LSTARTFDEDLSI1 /* Length FDE */
.LSTARTFDEDLSI1:
.long .LSTARTFDEDLSI1-.LSTARTFRAMEDLSI1 /* CIE pointer */
/* HACK: The dwarf2 unwind routines will subtract 1 from the
return address to get an address in the middle of the
presumed call instruction. Since we didn't get here via
a call, we need to include the nop before the real start
to make up for it. */
.long .LSTART_sigreturn-1-. /* PC-relative start address */
.long .LEND_sigreturn-.LSTART_sigreturn+1
.uleb128 0 /* Augmentation */
/* What follows are the instructions for the table generation.
We record the locations of each register saved. This is
complicated by the fact that the "CFA" is always assumed to
be the value of the stack pointer in the caller. This means
that we must define the CFA of this body of code to be the
saved value of the stack pointer in the sigcontext. Which
also means that there is no fixed relation to the other
saved registers, which means that we must use DW_CFA_expression
to compute their addresses. It also means that when we
adjust the stack with the popl, we have to do it all over again. */
#define do_cfa_expr(offset) \
.byte 0x0f; /* DW_CFA_def_cfa_expression */ \
.uleb128 1f-0f; /* length */ \
0: .byte 0x74; /* DW_OP_breg4 */ \
.sleb128 offset; /* offset */ \
.byte 0x06; /* DW_OP_deref */ \
1:
#define do_expr(regno, offset) \
.byte 0x10; /* DW_CFA_expression */ \
.uleb128 regno; /* regno */ \
.uleb128 1f-0f; /* length */ \
0: .byte 0x74; /* DW_OP_breg4 */ \
.sleb128 offset; /* offset */ \
1:
do_cfa_expr(IA32_SIGCONTEXT_sp+4)
do_expr(0, IA32_SIGCONTEXT_ax+4)
do_expr(1, IA32_SIGCONTEXT_cx+4)
do_expr(2, IA32_SIGCONTEXT_dx+4)
do_expr(3, IA32_SIGCONTEXT_bx+4)
do_expr(5, IA32_SIGCONTEXT_bp+4)
do_expr(6, IA32_SIGCONTEXT_si+4)
do_expr(7, IA32_SIGCONTEXT_di+4)
do_expr(8, IA32_SIGCONTEXT_ip+4)
.byte 0x42 /* DW_CFA_advance_loc 2 -- nop; popl eax. */
do_cfa_expr(IA32_SIGCONTEXT_sp)
do_expr(0, IA32_SIGCONTEXT_ax)
do_expr(1, IA32_SIGCONTEXT_cx)
do_expr(2, IA32_SIGCONTEXT_dx)
do_expr(3, IA32_SIGCONTEXT_bx)
do_expr(5, IA32_SIGCONTEXT_bp)
do_expr(6, IA32_SIGCONTEXT_si)
do_expr(7, IA32_SIGCONTEXT_di)
do_expr(8, IA32_SIGCONTEXT_ip)
.align 4
.LENDFDEDLSI1:
.long .LENDFDEDLSI2-.LSTARTFDEDLSI2 /* Length FDE */
.LSTARTFDEDLSI2:
.long .LSTARTFDEDLSI2-.LSTARTFRAMEDLSI1 /* CIE pointer */
/* HACK: See above wrt unwind library assumptions. */
.long .LSTART_rt_sigreturn-1-. /* PC-relative start address */
.long .LEND_rt_sigreturn-.LSTART_rt_sigreturn+1
.uleb128 0 /* Augmentation */
/* What follows are the instructions for the table generation.
We record the locations of each register saved. This is
slightly less complicated than the above, since we don't
modify the stack pointer in the process. */
do_cfa_expr(IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_sp)
do_expr(0, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_ax)
do_expr(1, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_cx)
do_expr(2, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_dx)
do_expr(3, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_bx)
do_expr(5, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_bp)
do_expr(6, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_si)
do_expr(7, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_di)
do_expr(8, IA32_RT_SIGFRAME_sigcontext-4 + IA32_SIGCONTEXT_ip)
.align 4
.LENDFDEDLSI2:
ud2a
CFI_ENDPROC
.size __kernel_rt_sigreturn,.-__kernel_rt_sigreturn
.previous

1
arch/x86/include/asm/dwarf2.h
View file

@ -20,6 +20,7 @@
#define CFI_RESTORE_STATE .cfi_restore_state
#define CFI_UNDEFINED .cfi_undefined
#define CFI_ESCAPE .cfi_escape
#define CFI_SIGNAL_FRAME .cfi_signal_frame
#ifndef BUILD_VDSO
/*

6
arch/x86/kernel/asm-offsets.c
View file

@ -63,8 +63,14 @@ static void __used common(void)
OFFSET(IA32_SIGCONTEXT_bp, sigcontext_32, bp);
OFFSET(IA32_SIGCONTEXT_sp, sigcontext_32, sp);
OFFSET(IA32_SIGCONTEXT_ip, sigcontext_32, ip);
OFFSET(IA32_SIGCONTEXT_es, sigcontext_32, es);
OFFSET(IA32_SIGCONTEXT_cs, sigcontext_32, cs);
OFFSET(IA32_SIGCONTEXT_ss, sigcontext_32, ss);
OFFSET(IA32_SIGCONTEXT_ds, sigcontext_32, ds);
OFFSET(IA32_SIGCONTEXT_flags, sigcontext_32, flags);
BLANK();
OFFSET(IA32_SIGFRAME_sigcontext, sigframe_ia32, sc);
OFFSET(IA32_RT_SIGFRAME_sigcontext, rt_sigframe_ia32, uc.uc_mcontext);
#endif