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[kernel] Allow for more than three syscall args

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The rcx register is used by the function call ABI for the 4th argument,
but is also clobbered by SYSCALL to hold the IP. The r10 register is
caller-saved but not part of the ABI, so stash rcx there when crossing
the syscall boundary.
This commit is contained in:
Justin C. Miller
2020-09-26 22:01:21 -07:00
parent 13aee1755e
commit 67ebc58812
2 changed files with 9 additions and 6 deletions
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8
src/kernel/syscall.s
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@@ -23,9 +23,11 @@ syscall_handler_prelude:
mov rbp, rsp mov rbp, rsp
; account for the hole in the sysv abi ; account for the hole in the sysv abi
; argument list since SYSCALL uses rcx ; argument list since SYSCALL uses rcx.
mov rcx, r8 ; r10 is non-preserved but not part of
mov r8, r9 ; the function call ABI, so the rcx arg
; was stashed there.
mov rcx, r10
push rbx push rbx
push r11 push r11

7
src/libraries/libc/arch/x86_64/syscalls.s
View File

@@ -5,9 +5,10 @@
mov rbp, rsp mov rbp, rsp
; args should already be in rdi, etc, but rcx will ; args should already be in rdi, etc, but rcx will
; get stomped, so shift args out one spot from rcx ; get stomped, so stash it in r10, which isn't a
mov r9, r8 ; callee-saved register, but also isn't used in the
mov r8, rcx ; function call ABI.
mov r10, rcx
mov rax, %2 mov rax, %2
syscall syscall