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

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

Author: H. Peter Anvin

arch/x86/entry/vdso/vdso32/sigreturn.S

@@ -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

arch/x86/include/asm/dwarf2.h

@@ -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
 	/*

arch/x86/kernel/asm-offsets.c

@@ -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