runtime: scan all writable program segments

Previously we used to scan between _edata and _end. This is not correct:
the .data section starts *before* _edata.

Fixing this would mean changing _edata to _etext, but that didn't quite
work either. It appears that there are inaccessible pages between _etext
and _end on ARM. Therefore, a different solution was needed.

What I've implemented is similar to Windows and MacOS: namely, finding
writable segments by parsing the program header of the currently running
program. It's a lot more verbose, but it should be correct on all
architectures. It probably also reduces the globals to scan to those
that _really_ need to be scanned.

This bug didn't result in issues in CI, but did result in a bug in the
recover branch: https://github.com/tinygo-org/tinygo/pull/2331. This
patch fixes this bug.

src/runtime/os_linux.go

@@ -24,22 +24,95 @@ const (
 	clock_MONOTONIC_RAW = 4
 )
 
-//go:extern _edata
-var globalsStartSymbol [0]byte
+// For the definition of the various header structs, see:
+// https://refspecs.linuxfoundation.org/elf/elf.pdf
+// Also useful:
+// https://en.wikipedia.org/wiki/Executable_and_Linkable_Format
+type elfHeader struct {
+	ident_magic      uint32
+	ident_class      uint8
+	ident_data       uint8
+	ident_version    uint8
+	ident_osabi      uint8
+	ident_abiversion uint8
+	_                [7]byte // reserved
+	filetype         uint16
+	machine          uint16
+	version          uint32
+	entry            uintptr
+	phoff            uintptr
+	shoff            uintptr
+	flags            uint32
+	ehsize           uint16
+	phentsize        uint16
+	phnum            uint16
+	shentsize        uint16
+	shnum            uint16
+	shstrndx         uint16
+}
 
-//go:extern _end
-var globalsEndSymbol [0]byte
+type elfProgramHeader64 struct {
+	_type  uint32
+	flags  uint32
+	offset uintptr
+	vaddr  uintptr
+	paddr  uintptr
+	filesz uintptr
+	memsz  uintptr
+	align  uintptr
+}
+
+type elfProgramHeader32 struct {
+	_type  uint32
+	offset uintptr
+	vaddr  uintptr
+	paddr  uintptr
+	filesz uintptr
+	memsz  uintptr
+	flags  uint32
+	align  uintptr
+}
+
+// ELF header of the currently running process.
+//
+//go:extern __ehdr_start
+var ehdr_start elfHeader
 
 // markGlobals marks all globals, which are reachable by definition.
-//
-// This implementation marks all globals conservatively and assumes it can use
-// linker-defined symbols for the start and end of the .data section.
+// It parses the ELF program header to find writable segments.
 func markGlobals() {
-	start := uintptr(unsafe.Pointer(&globalsStartSymbol))
-	end := uintptr(unsafe.Pointer(&globalsEndSymbol))
-	start = (start + unsafe.Alignof(uintptr(0)) - 1) &^ (unsafe.Alignof(uintptr(0)) - 1) // align on word boundary
+	// Relevant constants from the ELF specification.
+	// See: https://refspecs.linuxfoundation.org/elf/elf.pdf
+	const (
+		PT_LOAD = 1
+		PF_W    = 0x2 // program flag: write access
+	)
+
+	headerPtr := unsafe.Pointer(uintptr(unsafe.Pointer(&ehdr_start)) + ehdr_start.phoff)
+	for i := 0; i < int(ehdr_start.phnum); i++ {
+		// Look for a writable segment and scan its contents.
+		// There is a little bit of duplication here, which is unfortunate. But
+		// the alternative would be to put elfProgramHeader in separate files
+		// which is IMHO a lot uglier. If only the ELF spec was consistent
+		// between 32-bit and 64-bit...
+		if TargetBits == 64 {
+			header := (*elfProgramHeader64)(headerPtr)
+			if header._type == PT_LOAD && header.flags&PF_W != 0 {
+				start := header.vaddr
+				end := start + header.memsz
 				markRoots(start, end)
 			}
+		} else {
+			header := (*elfProgramHeader32)(headerPtr)
+			if header._type == PT_LOAD && header.flags&PF_W != 0 {
+				start := header.vaddr
+				end := start + header.memsz
+				markRoots(start, end)
+			}
+		}
+		headerPtr = unsafe.Pointer(uintptr(headerPtr) + uintptr(ehdr_start.phentsize))
+	}
+}
 
 //export getpagesize
 func libc_getpagesize() int