cmd/asm, cmd/link, runtime: introduce FuncInfo flag bits

The runtime traceback code has its own definition of which functions
mark the top frame of a stack, separate from the TOPFRAME bits that
exist in the assembly and are passed along in DWARF information.
It's error-prone and redundant to have two different sources of truth.
This CL provides the actual TOPFRAME bits to the runtime, so that
the runtime can use those bits instead of reinventing its own category.

This CL also adds a new bit, SPWRITE, which marks functions that
write directly to SP (anything but adding and subtracting constants).
Such functions must stop a traceback, because the traceback has no
way to rederive the SP on entry. Again, the runtime has its own definition
which is mostly correct, but also missing some functions. During ordinary
goroutine context switches, such functions do not appear on the stack,
so the incompleteness in the runtime usually doesn't matter.
But profiling signals can arrive at any moment, and the runtime may
crash during traceback if it attempts to unwind an SP-writing frame
and gets out-of-sync with the actual stack. The runtime contains code
to try to detect likely candidates but again it is incomplete.
Deriving the SPWRITE bit automatically from the actual assembly code
provides the complete truth, and passing it to the runtime lets the
runtime use it.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I227f53b23ac5b3dabfcc5e8ee3f00df4e113cf58
Reviewed-on: https://go-review.googlesource.com/c/go/+/288800
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 12 insertions, 25 deletions

diff --git a/src/cmd/link/internal/loader/loader.go b/src/cmd/link/internal/loader/loader.go
index 98c2131c2b..68dc3de273 100644
--- a/src/cmd/link/internal/loader/loader.go
+++ b/src/cmd/link/internal/loader/loader.go
@@ -241,7 +241,6 @@ type Loader struct {
 	attrExternal         Bitmap // external symbols, indexed by ext sym index
 
 	attrReadOnly         map[Sym]bool     // readonly data for this sym
-	attrTopFrame         map[Sym]struct{} // top frame symbols
 	attrSpecial          map[Sym]struct{} // "special" frame symbols
 	attrCgoExportDynamic map[Sym]struct{} // "cgo_export_dynamic" symbols
 	attrCgoExportStatic  map[Sym]struct{} // "cgo_export_static" symbols
@@ -349,7 +348,6 @@ func NewLoader(flags uint32, elfsetstring elfsetstringFunc, reporter *ErrorRepor
 		plt:                  make(map[Sym]int32),
 		got:                  make(map[Sym]int32),
 		dynid:                make(map[Sym]int32),
-		attrTopFrame:         make(map[Sym]struct{}),
 		attrSpecial:          make(map[Sym]struct{}),
 		attrCgoExportDynamic: make(map[Sym]struct{}),
 		attrCgoExportStatic:  make(map[Sym]struct{}),
@@ -1009,24 +1007,6 @@ func (l *Loader) SetAttrExternal(i Sym, v bool) {
 	}
 }
 
-// AttrTopFrame returns true for a function symbol that is an entry
-// point, meaning that unwinders should stop when they hit this
-// function.
-func (l *Loader) AttrTopFrame(i Sym) bool {
-	_, ok := l.attrTopFrame[i]
-	return ok
-}
-
-// SetAttrTopFrame sets the "top frame" property for a symbol (see
-// AttrTopFrame).
-func (l *Loader) SetAttrTopFrame(i Sym, v bool) {
-	if v {
-		l.attrTopFrame[i] = struct{}{}
-	} else {
-		delete(l.attrTopFrame, i)
-	}
-}
-
 // AttrSpecial returns true for a symbols that do not have their
 // address (i.e. Value) computed by the usual mechanism of
 // data.go:dodata() & data.go:address().
@@ -1905,7 +1885,11 @@ func (fi *FuncInfo) Locals() int {
 }
 
 func (fi *FuncInfo) FuncID() objabi.FuncID {
-	return objabi.FuncID((*goobj.FuncInfo)(nil).ReadFuncID(fi.data))
+	return (*goobj.FuncInfo)(nil).ReadFuncID(fi.data)
+}
+
+func (fi *FuncInfo) FuncFlag() objabi.FuncFlag {
+	return (*goobj.FuncInfo)(nil).ReadFuncFlag(fi.data)
 }
 
 func (fi *FuncInfo) Pcsp() Sym {
@@ -1992,6 +1976,13 @@ func (fi *FuncInfo) File(k int) goobj.CUFileIndex {
 	return (*goobj.FuncInfo)(nil).ReadFile(fi.data, fi.lengths.FileOff, uint32(k))
 }
 
+// TopFrame returns true if the function associated with this FuncInfo
+// is an entry point, meaning that unwinders should stop when they hit
+// this function.
+func (fi *FuncInfo) TopFrame() bool {
+	return (fi.FuncFlag() & objabi.FuncFlag_TOPFRAME) != 0
+}
+
 type InlTreeNode struct {
 	Parent   int32
 	File     goobj.CUFileIndex
@@ -2151,9 +2142,6 @@ func (st *loadState) preloadSyms(r *oReader, kind int) {
 		}
 		gi := st.addSym(name, v, r, i, kind, osym)
 		r.syms[i] = gi
-		if osym.TopFrame() {
-			l.SetAttrTopFrame(gi, true)
-		}
 		if osym.Local() {
 			l.SetAttrLocal(gi, true)
 		}
@@ -2411,7 +2399,6 @@ func (l *Loader) CopyAttributes(src Sym, dst Sym) {
 		// when copying attributes from a dupOK ABI wrapper symbol to
 		// the real target symbol (which may not be marked dupOK).
 	}
-	l.SetAttrTopFrame(dst, l.AttrTopFrame(src))
 	l.SetAttrSpecial(dst, l.AttrSpecial(src))
 	l.SetAttrCgoExportDynamic(dst, l.AttrCgoExportDynamic(src))
 	l.SetAttrCgoExportStatic(dst, l.AttrCgoExportStatic(src))