# movq 16(%rsp),%rsi
# endif
# ret
+#
+#################################################
+# Unlike on Unix systems(*) lack of Win64 stack unwinding information
+# has undesired side-effect at run-time: if an exception is raised in
+# assembler subroutine such as those in question (basically we're
+# referring to segmentation violations caused by malformed input
+# parameters), the application is briskly terminated without invoking
+# any exception handlers, most notably without generating memory dump
+# or any user notification whatsoever. This poses a problem. It's
+# possible to address it by registering custom language-specific
+# handler that would restore processor context to the state at
+# subroutine entry point and return "exception is not handled, keep
+# unwinding" code. Writing such handler can be a challenge... But it's
+# doable, though requires certain coding convention. Consider following
+# snippet:
+#
+# function:
+# movq %rsp,%rax # copy rsp to volatile register
+# pushq %r15 # save non-volatile registers
+# pushq %rbx
+# pushq %rbp
+# movq %rsp,%r11
+# subq %rdi,%r11 # prepare [variable] stack frame
+# andq $-64,%r11
+# movq %rax,0(%r11) # check for exceptions
+# movq %r11,%rsp # allocate [variable] stack frame
+# movq %rax,0(%rsp) # save original rsp value
+# magic_point:
+# ...
+# movq 0(%rsp),%rcx # pull original rsp value
+# movq -24(%rcx),%rbp # restore non-volatile registers
+# movq -16(%rcx),%rbx
+# movq -8(%rcx),%r15
+# movq %rcx,%rsp # restore original rsp
+# ret
+#
+# The key is that up to magic_point copy of original rsp value remains
+# in chosen volatile register and no non-volatile register, except for
+# rsp, is modified. While past magic_point rsp remains constant till
+# the very end of the function. In this case custom language-specific
+# exception handler would look like this:
+#
+# EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame,
+# CONTEXT *context,DISPATCHER_CONTEXT *disp)
+# { ULONG64 *rsp;
+# if (context->Rip<magic_point)
+# rsp = (ULONG64 *)context->Rax;
+# else
+# { rsp = ((ULONG64 **)context->Rsp)[0];
+# context->Rbp = rsp[-3];
+# context->Rbx = rsp[-2];
+# context->R15 = rsp[-1];
+# }
+# context->Rsp = (ULONG64)rsp;
+# context->Rdi = rsp[1];
+# context->Rsi = rsp[2];
+#
+# memcpy (disp->ContextRecord,context,sizeof(CONTEXT));
+# RtlVirtualUnwind(UNW_FLAG_NHANDLER,disp->ImageBase,
+# dips->ControlPc,disp->FunctionEntry,disp->ContextRecord,
+# &disp->HandlerData,&disp->EstablisherFrame,NULL);
+# return ExceptionContinueSearch;
+# }
+#
+# It's appropriate to implement this handler in assembler, directly in
+# function's module. In order to do that one has to know members'
+# offsets in CONTEXT and DISPATCHER_CONTEXT structures and some constant
+# values. Here they are:
+#
+# CONTEXT.Rax 120
+# CONTEXT.Rcx 128
+# CONTEXT.Rdx 136
+# CONTEXT.Rbx 144
+# CONTEXT.Rsp 152
+# CONTEXT.Rbp 160
+# CONTEXT.Rsi 168
+# CONTEXT.Rdi 176
+# CONTEXT.R8 184
+# CONTEXT.R9 192
+# CONTEXT.R10 200
+# CONTEXT.R11 208
+# CONTEXT.R12 216
+# CONTEXT.R13 224
+# CONTEXT.R14 232
+# CONTEXT.R15 240
+# CONTEXT.Rip 248
+# sizeof(CONTEXT) 1232
+# DISPATCHER_CONTEXT.ControlPc 0
+# DISPATCHER_CONTEXT.ImageBase 8
+# DISPATCHER_CONTEXT.FunctionEntry 16
+# DISPATCHER_CONTEXT.EstablisherFrame 24
+# DISPATCHER_CONTEXT.TargetIp 32
+# DISPATCHER_CONTEXT.ContextRecord 40
+# DISPATCHER_CONTEXT.LanguageHandler 48
+# DISPATCHER_CONTEXT.HandlerData 56
+# UNW_FLAG_NHANDLER 0
+# ExceptionContinueSearch 1
+#
+# UNWIND_INFO structure for .xdata segment would be
+# DB 9,0,0,0
+# DD imagerel handler
+# denoting exception handler for a function with zero-length prologue,
+# no stack frame or frame register.
+#
+# P.S. Attentive reader can notice that effectively no exceptions are
+# expected in "gear" prologue and epilogue [discussed in "ABI
+# cross-reference" above]. No, there are not. This is because if
+# memory area used by them was subject to segmentation violation,
+# then exception would be raised upon call to our function and be
+# accounted to caller and unwound from its frame, which is not a
+# problem.
+#
+# (*) Note that we're talking about run-time, not debug-time. Lack of
+# unwind information makes debugging hard on both Windows and
+# Unix. "Unlike" referes to the fact that on Unix signal handler
+# will always be invoked, core dumped and appropriate exit code
+# returned to parent (for user notification).
projects / openssl.git / blobdiff