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Kernel: Comment the living daylights out of signal trampoline/sigreturn

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Mere mortals like myself cannot understand more than two lines of
assembly without a million comments explaining what's happening, so do
that and make sure no one has to go on a wild stack state chase when
hacking on these.
This commit is contained in:
Ali Mohammad Pur 2022-02-25 20:37:09 +03:30 • committed by Andreas Kling
parent 7238c946f0
commit 585054d68b
2 changed files with 40 additions and 12 deletions
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14
Kernel/Syscalls/sigaction.cpp
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@ -84,9 +84,14 @@ ErrorOr<FlatPtr> Process::sys$sigreturn([[maybe_unused]] RegisterState& register
SmapDisabler disabler;
#if ARCH(I386)
// Stack state (created by the signal trampoline):
// ret flags, ret ip, register dump,
// signal mask, signal, handler (alignment = 16),
// 0, ebp, eax
// Here, we restore the state pushed by dispatch signal and asm_signal_trampoline.
u32* stack_ptr = (u32*)registers.userspace_esp;
u32 smuggled_eax = *stack_ptr;
FlatPtr* stack_ptr = bit_cast<FlatPtr*>(registers.userspace_esp);
FlatPtr smuggled_eax = *stack_ptr;
// pop the stored eax, ebp, return address, handler and signal code
stack_ptr += 5;
@ -107,6 +112,11 @@ ErrorOr<FlatPtr> Process::sys$sigreturn([[maybe_unused]] RegisterState& register
registers.userspace_esp = registers.esp;
return smuggled_eax;
#else
// Stack state (created by the signal trampoline):
// ret flags, ret ip, register dump,
// signal mask, signal, handler (alignment = 16),
// 0, ebp, eax
// Here, we restore the state pushed by dispatch signal and asm_signal_trampoline.
FlatPtr* stack_ptr = (FlatPtr*)registers.userspace_rsp;
FlatPtr smuggled_rax = *stack_ptr;