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Rearrange makecontext and add lots of comments

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* exceptions.cc (makecontext): Rearrange order of initialization and
        document at great length.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen
2015-07-18 12:35:23 +02:00
parent b2df1577c7
commit 877b02be08
2 changed files with 42 additions and 12 deletions
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5
winsup/cygwin/ChangeLog
View File

@@ -1,3 +1,8 @@
2015-07-17 Corinna Vinschen <corinna@vinschen.de>
* exceptions.cc (makecontext): Rearrange order of initialization and
document at great length.
2015-07-17 Corinna Vinschen <corinna@vinschen.de>
* exceptions.cc (__unwind_single_frame): Define empty macro on i686.

49
winsup/cygwin/exceptions.cc
View File

@@ -2030,22 +2030,31 @@ makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
uintptr_t *sp;
va_list ap;
sp = (uintptr_t *) ((uintptr_t) ucp->uc_stack.ss_sp
+ ucp->uc_stack.ss_size);
/* Initialize sp to the top of the stack. */
sp = (uintptr_t *) ((uintptr_t) ucp->uc_stack.ss_sp + ucp->uc_stack.ss_size);
/* Subtract slots required for arguments and the pointer to uc_link. */
sp -= (argc + 1);
/* Align. */
sp = (uintptr_t *) ((uintptr_t) sp & ~0xf);
/* Subtract one slot for setting the return address. */
--sp;
/* Set return address to the trampolin function __cont_link_context. */
sp[0] = (uintptr_t) __cont_link_context;
sp[argc + 1] = (uintptr_t) ucp->uc_link;
#ifdef __x86_64__
ucp->uc_mcontext.rip = (uint64_t) func;
ucp->uc_mcontext.rbx = (uint64_t) (sp + argc + 1);
ucp->uc_mcontext.rsp = (uint64_t) sp;
#else
ucp->uc_mcontext.eip = (uint32_t) func;
ucp->uc_mcontext.ebx = (uint32_t) (sp + argc + 1);
ucp->uc_mcontext.esp = (uint32_t) sp;
#endif
/* Fetch arguments and store them on the stack.
x86_64 only:
- Store first four args in the AMD64 ABI arg registers.
- Note that the stack is not short by these four register args. The
reason is the shadow space for these regs required by the AMD64 ABI.
- The definition of makecontext only allows for "int" sized arguments to
func, 32 bit, likely for historical reasons. However, the argument
slots on x86_64 are 64 bit anyway, so we can fetch and store the args
as 64 bit values, and func can request 64 bit args without violating
the definition. This potentially allows porting 32 bit applications
providing pointer values to func without additional porting effort. */
va_start (ap, argc);
for (int i = 0; i < argc; ++i)
#ifdef __x86_64__
@@ -2071,4 +2080,20 @@ makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
sp[i + 1] = va_arg (ap, uintptr_t);
#endif
va_end (ap);
/* Store pointer to uc_link at the top of the stack. */
sp[argc + 1] = (uintptr_t) ucp->uc_link;
/* Last but not least set the register in the context at ucp so that a
subsequent setcontext or swapcontext picks up the right values:
- Set rip/eip to the target function.
- Set rsp/esp to the just computed stack pointer value.
- Set rbx/ebx to the address of the pointer to uc_link. */
#ifdef __x86_64__
ucp->uc_mcontext.rip = (uint64_t) func;
ucp->uc_mcontext.rsp = (uint64_t) sp;
ucp->uc_mcontext.rbx = (uint64_t) (sp + argc + 1);
#else
ucp->uc_mcontext.eip = (uint32_t) func;
ucp->uc_mcontext.esp = (uint32_t) sp;
ucp->uc_mcontext.ebx = (uint32_t) (sp + argc + 1);
#endif
}