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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
Notes: sideshowbarker 2024-07-17 17:57:17 +09:00
2 changed files with 40 additions and 12 deletions
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38
Kernel/Process.cpp
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@ -297,16 +297,25 @@ void signal_trampoline_dummy()
".intel_syntax noprefix\n"
".globl asm_signal_trampoline\n"
"asm_signal_trampoline:\n"
// stack state: ret flags, ret ip, register dump, signal mask, signal, handler (alignment = 16), 0
// save ebp
"push ebp\n"
"mov ebp, esp\n"
"push eax\n" // we have to store eax 'cause it might be the return value from a syscall
"sub esp, 4\n" // align the stack to 16 bytes
"mov eax, [ebp+12]\n" // push the signal code
// we have to save eax 'cause it might be the return value from a syscall
"push eax\n"
"call [ebp+8]\n" // call the signal handler
// align the stack to 16 bytes (as our current offset is 12 from the fake return addr, saved ebp and saved eax)
"sub esp, 4\n"
// push the signal code
"mov eax, [ebp+12]\n"
"push eax\n"
// call the signal handler
"call [ebp+8]\n"
// Unroll stack back to the saved eax
"add esp, 8\n"
// syscall SC_sigreturn
"mov eax, %P0\n"
"int 0x82\n" // sigreturn syscall
"int 0x82\n"
".globl asm_signal_trampoline_end\n"
"asm_signal_trampoline_end:\n"
".att_syntax" ::"i"(Syscall::SC_sigreturn));
@ -320,15 +329,24 @@ void signal_trampoline_dummy()
".intel_syntax noprefix\n"
".globl asm_signal_trampoline\n"
"asm_signal_trampoline:\n"
// stack state: ret flags, ret ip, register dump, signal mask, signal, handler (alignment = 16), 0
// save rbp
"push rbp\n"
"mov rbp, rsp\n"
"push rax\n" // we have to store rax 'cause it might be the return value from a syscall
"sub rsp, 8\n" // align the stack to 16 bytes
"mov rdi, [rbp+24]\n" // push the signal code
"call [rbp+16]\n" // call the signal handler
// we have to save rax 'cause it might be the return value from a syscall
"push rax\n"
// align the stack to 16 bytes (our offset is 24 bytes from the fake return addr, saved rbp and saved rax).
"sub rsp, 8\n"
// push the signal code
"mov rdi, [rbp+24]\n"
// call the signal handler
"call [rbp+16]\n"
// unroll stack back to the saved rax
"add rsp, 8\n"
// syscall SC_sigreturn
"mov rax, %P0\n"
"int 0x82\n" // sigreturn syscall
"int 0x82\n"
".globl asm_signal_trampoline_end\n"
"asm_signal_trampoline_end:\n"
".att_syntax" ::"i"(Syscall::SC_sigreturn));