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In a kernel or protected build vfork() is reached through a system call, so
the return address and stack pointer that the entry point in
arm64_fork_func.S can snapshot for itself belong to the kernel-side stub, not
to the caller. A child built from that snapshot resumes at a kernel address
on a kernel stack. This is why arm64 selected the fork family only for the
flat build.
Record what the caller was actually doing instead. arm64_sync_exc passes the
exception frame to dispatch_syscall() in x7 -- x0-x6 carry the call number and
its six parameters, so x7 is free -- and dispatch_syscall() stores it in
xcp.sregs, mirroring what riscv_swint.c does.
arm64_fork() then chooses where the caller's registers live:
arm64_fork_syscall() when TCB_FLAG_SYSCALL is set, rebuilding the child
from xcp.sregs so that it returns from the very same
SVC as the parent;
arm64_fork_direct() otherwise, which is the flat build and any kernel
thread that calls the entry point as a plain function.
The stack copy and the relocation of pointers into it are shared by both
paths in arm64_fork_stack() and arm64_fork_reloc().
With that in place ARCH_ARM64 can select ARCH_HAVE_VFORK unconditionally.
Verified on qemu-armv8a:knsh (BUILD_KERNEL), qemu-armv8a:nsh (BUILD_FLAT) and
qemu-armv8a:citest_smp under qemu-system-aarch64: ostest's vfork_test passes
on all three, and it was absent from knsh before the change. The protected
configurations are build-verified only (fvp-armv8r:pnsh), there being no
emulator for them here.
Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
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