Fix stack pointer retrieval in jl_backtrace_from_here (#42585)

Co-authored-by: Jameson Nash <[email protected]>

src/stackwalk.c

@@ -114,8 +114,6 @@ static int jl_unw_stepn(bt_cursor_t *cursor, jl_bt_element_t *bt_data, size_t *b
                 from_signal_handler = 0;
                 continue;
             }
-            if (sp)
-                sp[n] = thesp;
             // For the purposes of looking up debug info for functions, we want
             // to harvest addresses for the *call* instruction `call_ip` during
             // stack walking.  However, this information isn't directly
@@ -168,6 +166,8 @@ static int jl_unw_stepn(bt_cursor_t *cursor, jl_bt_element_t *bt_data, size_t *b
                 }
             }
             bt_entry->uintptr = call_ip;
+            if (sp)
+                sp[n] = thesp;
             n++;
         }
         // NOTE: if we have some pgcstack entries remaining (because the
@@ -259,8 +259,8 @@ JL_DLLEXPORT jl_value_t *jl_backtrace_from_here(int returnsp, int skip)
             jl_array_grow_end(ip, maxincr);
             uintptr_t *sp_ptr = NULL;
             if (returnsp) {
-                sp_ptr = (uintptr_t*)jl_array_data(sp) + offset;
                 jl_array_grow_end(sp, maxincr);
+                sp_ptr = (uintptr_t*)jl_array_data(sp) + offset;
             }
             size_t size_incr = 0;
             have_more_frames = jl_unw_stepn(&cursor, (jl_bt_element_t*)jl_array_data(ip) + offset,

test/backtrace.jl

@@ -258,3 +258,88 @@ let code = """
     @test occursin("InterpreterIP in top-level CodeInfo for Main.A", bt_str)
 end
 
+"""
+    _reformat_sp(bt_data...) -> sp::Vector{Ptr{Cvoid}}
+
+Convert the output `bt_data` of `jl_backtrace_from_here` with `returnsp` flag set to a
+vector of valid stack pointers `sp`; i.e., `sp` is a subset of `bt_data[3]`.
+
+See also `Base._reformat_bt`.
+"""
+function _reformat_sp(
+    bt_raw::Array{Ptr{Cvoid},1},
+    bt2::Array{Any,1},
+    sp_raw::Array{Ptr{Cvoid},1},
+)
+    bt = Base._reformat_bt(bt_raw, bt2)
+    sp = empty!(similar(sp_raw))
+    i = j = 0
+    while true
+        # Advance `i` such that `bt[i] isa Ptr{Cvoid}` (native pointer).
+        local ip
+        while true
+            if i == lastindex(bt)
+                return sp
+            end
+            i += 1
+            x = bt[i]
+            if x isa Ptr{Cvoid}
+                ip = x
+                break
+            end
+        end
+        # Advance `j` such that `bt_raw[j] == bt[i]` to find a valid stack pointer.
+        while true
+            if j == lastindex(bt_raw)
+                return sp
+            end
+            j += 1
+            if bt_raw[j] == ip
+                push!(sp, sp_raw[j])
+                break
+            end
+        end
+    end
+end
+
+"""
+    withframeaddress(f)
+
+Call function `f` with an address `ptr::Ptr{Cvoid}` of an independent frame
+immediately outer to `f`.
+"""
+withframeaddress
+@eval @noinline function withframeaddress(f)
+    sp = Core.Intrinsics.llvmcall(
+        ($"""
+        declare i8* @llvm.frameaddress(i32)
+        define private i$(Sys.WORD_SIZE) @frameaddr() {
+            %1 = call i8* @llvm.frameaddress(i32 0)
+            %2 = ptrtoint i8* %1 to i$(Sys.WORD_SIZE)
+            ret i$(Sys.WORD_SIZE) %2
+        }""", "frameaddr"),
+        UInt,
+        Tuple{},
+    )
+    @noinline f(Ptr{Cvoid}(sp))
+end
+
+function sandwiched_backtrace()
+    local ptr1, ptr2, bt
+    withframeaddress() do p1
+        ptr1 = p1
+        bt = ccall(:jl_backtrace_from_here, Ref{Base.SimpleVector}, (Cint, Cint), true, 0)
+        withframeaddress() do p2
+            ptr2 = p2
+        end
+    end
+    return ptr1, ptr2, bt
+end
+
+@testset "stack pointers" begin
+    ptr1, ptr2, bt_data = sandwiched_backtrace()
+    sp = _reformat_sp(bt_data...)
+    @test ptr2 < sp[2]
+    @test sp[1] < ptr1
+    @test all(diff(Int128.(UInt.(sp))) .> 0)
+end