[libc++abi] Fix lpStart adjustment for exceptions table (#72727)

When lpStartEncoding is different from DW_EH_PE_omit, lpStart can be set
to zero which is a valid base address for landing pads. Such base value
is useful when landing pads are placed in different sections.

Fixes #72582.

Author: maksfb

libcxxabi/src/cxa_personality.cpp

@@ -660,10 +660,9 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
     //       dwarf emission
     // Parse LSDA header.
     uint8_t lpStartEncoding = *lsda++;
-    const uint8_t* lpStart =
-        (const uint8_t*)readEncodedPointer(&lsda, lpStartEncoding, base);
-    if (lpStart == 0)
-        lpStart = (const uint8_t*)funcStart;
+    const uint8_t* lpStart = lpStartEncoding == DW_EH_PE_omit
+                                 ? (const uint8_t*)funcStart
+                                 : (const uint8_t*)readEncodedPointer(&lsda, lpStartEncoding, base);
     uint8_t ttypeEncoding = *lsda++;
     if (ttypeEncoding != DW_EH_PE_omit)
     {

libcxxabi/test/native/x86_64/lpstart-zero.pass.sh.s

@@ -0,0 +1,109 @@
+# RUN: %{cxx} %{flags} %s %{link_flags} -no-pie -o %t.exe
+# RUN: %t.exe
+
+# REQUIRES: linux && target={{x86_64-.+}}
+# UNSUPPORTED: no-exceptions
+
+## Check that libc++abi works correctly when LPStart address is explicitly set
+## to zero.
+
+## This file is generated from the following C++ source code.
+##
+## ```
+## int main() {
+##   try {
+##     throw 42;
+##   } catch (...) {
+##     return 0;
+##   }
+##   return 1;
+## }
+## ```
+## The exception table is modified to use udata4 encoding for LPStart and
+## sdata4 encoding for call sites.
+
+	.text
+	.globl	main                            # -- Begin function main
+	.p2align	4, 0x90
+	.type	main,@function
+main:                                   # @main
+.Lfunc_begin0:
+	.cfi_startproc
+	.globl	__gxx_personality_v0
+	.cfi_personality 3, __gxx_personality_v0
+	.cfi_lsda 27, .Lexception0
+# %bb.0:                                # %entry
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset %rbp, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register %rbp
+	subq	$32, %rsp
+	movl	$0, -4(%rbp)
+	movl	$4, %edi
+	callq	__cxa_allocate_exception@PLT
+	movq	%rax, %rdi
+	movl	$42, (%rdi)
+.Ltmp0:
+	movq	_ZTIi@GOTPCREL(%rip), %rsi
+	xorl	%eax, %eax
+	movl	%eax, %edx
+	callq	__cxa_throw@PLT
+.Ltmp1:
+	jmp	.LBB0_4
+.LBB0_1:                                # %lpad
+.Ltmp2:
+	movq	%rax, %rcx
+	movl	%edx, %eax
+	movq	%rcx, -16(%rbp)
+	movl	%eax, -20(%rbp)
+# %bb.2:                                # %catch
+	movq	-16(%rbp), %rdi
+	callq	__cxa_begin_catch@PLT
+	movl	$0, -4(%rbp)
+	callq	__cxa_end_catch@PLT
+# %bb.3:                                # %return
+	movl	-4(%rbp), %eax
+	addq	$32, %rsp
+	popq	%rbp
+	.cfi_def_cfa %rsp, 8
+	retq
+.LBB0_4:                                # %unreachable
+.Lfunc_end0:
+	.size	main, .Lfunc_end0-main
+	.cfi_endproc
+
+	.section	.gcc_except_table,"a",@progbits
+	.p2align	2, 0x0
+GCC_except_table0:
+.Lexception0:
+	.byte	3                               # @LPStart Encoding = udata4
+  .long 0
+	.byte	155                             # @TType Encoding = indirect pcrel sdata4
+	.uleb128 .Lttbase0-.Lttbaseref0
+.Lttbaseref0:
+	.byte	11                              # Call site Encoding = udata4
+	.uleb128 .Lcst_end0-.Lcst_begin0
+.Lcst_begin0:
+	.long .Lfunc_begin0-.Lfunc_begin0    # >> Call Site 1 <<
+	.long .Ltmp0-.Lfunc_begin0           #   Call between .Lfunc_begin0 and .Ltmp0
+	.long	0                               #     has no landing pad
+	.byte	0                               #   On action: cleanup
+	.long .Ltmp0-.Lfunc_begin0           # >> Call Site 2 <<
+	.long .Ltmp1-.Ltmp0                  #   Call between .Ltmp0 and .Ltmp1
+  .long .Ltmp2
+	.byte	1                               #   On action: 1
+	.long .Ltmp1-.Lfunc_begin0           # >> Call Site 3 <<
+	.long .Lfunc_end0-.Ltmp1             #   Call between .Ltmp1 and .Lfunc_end0
+	.long	0                               #     has no landing pad
+	.byte	0                               #   On action: cleanup
+.Lcst_end0:
+	.byte	1                               # >> Action Record 1 <<
+                                        #   Catch TypeInfo 1
+	.byte	0                               #   No further actions
+	.p2align	2, 0x0
+                                        # >> Catch TypeInfos <<
+	.long	0                               # TypeInfo 1
+.Lttbase0:
+	.p2align	2, 0x0
+                                        # -- End function