diff options
Diffstat (limited to 'common-user/host/x86_64/safe-syscall.inc.S')
| -rw-r--r-- | common-user/host/x86_64/safe-syscall.inc.S | 94 |
1 files changed, 94 insertions, 0 deletions
diff --git a/common-user/host/x86_64/safe-syscall.inc.S b/common-user/host/x86_64/safe-syscall.inc.S new file mode 100644 index 0000000000..84fed206f9 --- /dev/null +++ b/common-user/host/x86_64/safe-syscall.inc.S @@ -0,0 +1,94 @@ +/* + * safe-syscall.inc.S : host-specific assembly fragment + * to handle signals occurring at the same time as system calls. + * This is intended to be included by common-user/safe-syscall.S + * + * Copyright (C) 2015 Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk> + * + * This work is licensed under the terms of the GNU GPL, version 2 or later. + * See the COPYING file in the top-level directory. + */ + + .global safe_syscall_base + .global safe_syscall_start + .global safe_syscall_end + .type safe_syscall_base, @function + + /* This is the entry point for making a system call. The calling + * convention here is that of a C varargs function with the + * first argument an 'int *' to the signal_pending flag, the + * second one the system call number (as a 'long'), and all further + * arguments being syscall arguments (also 'long'). + */ +safe_syscall_base: + .cfi_startproc + /* This saves a frame pointer and aligns the stack for the syscall. + * (It's unclear if the syscall ABI has the same stack alignment + * requirements as the userspace function call ABI, but better safe than + * sorry. Appendix A2 of http://www.x86-64.org/documentation/abi.pdf + * does not list any ABI differences regarding stack alignment.) + */ + push %rbp + .cfi_adjust_cfa_offset 8 + .cfi_rel_offset rbp, 0 + + /* + * The syscall calling convention isn't the same as the C one: + * we enter with rdi == &signal_pending + * rsi == syscall number + * rdx, rcx, r8, r9, (stack), (stack) == syscall arguments + * and return the result in rax + * and the syscall instruction needs + * rax == syscall number + * rdi, rsi, rdx, r10, r8, r9 == syscall arguments + * and returns the result in rax + * Shuffle everything around appropriately. + * Note that syscall will trash rcx and r11. + */ + mov %rsi, %rax /* syscall number */ + mov %rdi, %rbp /* signal_pending pointer */ + /* and the syscall arguments */ + mov %rdx, %rdi + mov %rcx, %rsi + mov %r8, %rdx + mov %r9, %r10 + mov 16(%rsp), %r8 + mov 24(%rsp), %r9 + + /* This next sequence of code works in conjunction with the + * rewind_if_safe_syscall_function(). If a signal is taken + * and the interrupted PC is anywhere between 'safe_syscall_start' + * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'. + * The code sequence must therefore be able to cope with this, and + * the syscall instruction must be the final one in the sequence. + */ +safe_syscall_start: + /* if signal_pending is non-zero, don't do the call */ + cmpl $0, (%rbp) + jnz 2f + syscall +safe_syscall_end: + /* code path for having successfully executed the syscall */ + cmp $-4095, %rax + jae 0f + pop %rbp + .cfi_remember_state + .cfi_def_cfa_offset 8 + .cfi_restore rbp + ret + .cfi_restore_state + +0: neg %eax + jmp 1f + + /* code path when we didn't execute the syscall */ +2: mov $QEMU_ERESTARTSYS, %eax + + /* code path setting errno */ +1: pop %rbp + .cfi_def_cfa_offset 8 + .cfi_restore rbp + jmp safe_syscall_set_errno_tail + .cfi_endproc + + .size safe_syscall_base, .-safe_syscall_base |