Re: [PATCH v2 4/9] bsd-user/syscall: Replace alloca() by g_new()

[Peter Maydell]

On Thu, 6 May 2021 at 15:57, Warner Losh <imp@bsdimp.com> wrote:
> malloc, on the other hand, involves taking out a number of mutexes
> and similar things to obtain the memory, which may not necessarily
> be safe in all the contexts system calls can be called from. System
> calls are, typically, async safe and can be called from signal handlers.
> alloca() is async safe, while malloc() is not. So changing the calls
> from alloca to malloc makes calls to system calls in signal handlers
> unsafe and potentially introducing buggy behavior as a result.

malloc() should definitely be fine in this context. The syscall
emulation is called after the cpu_loop() in bsd-user has called
cpu_exec(). cpu_exec() calls into the JIT, which will malloc
all over the place if it needs to in the course of JITting things.

This code should never be being called from a (host) signal handler.
In upstream the signal handling code for bsd-user appears to
be missing entirely. For linux-user when we take a host signal
we merely arrange for the guest to take a guest signal, we
don't try to execute guest code directly from the host handler.

(There are some pretty hairy races in emulated signal handling:
we did a big overhaul of the linux-user code in that area a
while back. If your bsd-user code doesn't have the 'safe_syscall'
stuff it probably needs it. But that's more about races between
"guest code wants to execute a syscall" and "incoming signal"
where we could fail to exit EINTR from an emulated syscall if
the signal arrives in a bad window.)

thanks
-- PMM