d0f7acf9eb
An x86-64 operating system built from xv6, now largely rewritten: SMP, a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF rasteriser, and a Linux-numbered syscall ABI. No history before this point -- the tree was never under version control. This commit exists so the rename that follows has something to undo to.
44 lines
1.8 KiB
ArmAsm
44 lines
1.8 KiB
ArmAsm
/* SPDX-License-Identifier: Apache-2.0
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* Schunix — process entry point.
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*
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* This has to be assembly, and the reason is worth recording.
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*
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* exec() hands a new process its arguments in registers, not on the stack:
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* kernel/fs/exec.c sets proc->tf->rdi = argc and proc->tf->rsi = argv. Those
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* are already the first two SysV argument registers, so _start's entire job
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* is to call main and then exit — it must not touch %rdi or %rsi at all.
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*
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* It used to be written in C, capturing the incoming registers with GCC's
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*
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* register int argc_reg asm("rdi");
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*
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* That is not what local register variables mean. Both GCC and clang only
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* guarantee the binding when the variable is used as an operand of an inline
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* asm statement; reading one to pick up an incoming argument is undefined.
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* GCC happened to do the useful thing. clang compiled it to
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*
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* movl -0x4(%rbp), %eax # two uninitialised stack slots
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* movq -0x10(%rbp), %rax
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*
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* and every program was handed a garbage argc and argv. Nothing crashed --
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* init, shmtest and socktest take no arguments and passed their whole test
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* suites -- so the only symptom was `inputtest -n 6` quietly ignoring its
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* limit. A compiler-dependent way to read registers has no business being in
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* C when four instructions of assembly are exactly right.
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*/
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.text
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.globl _start
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_start:
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xorq %rbp, %rbp /* end the frame-pointer chain here */
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/* SysV wants %rsp 16-byte aligned immediately before a call, so that the
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* callee sees %rsp+8 aligned once the return address is pushed. Rounding
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* down is safe: argc and argv arrive in registers, so nothing below is
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* needed. */
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andq $-16, %rsp
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callq main /* main(%rdi = argc, %rsi = argv) */
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callq exit /* exit() is noreturn, but be certain */
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1: hlt
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jmp 1b
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