From: Peter Maydell <peter.maydell@linaro.org>
To: Warner Losh <imp@bsdimp.com>
Cc: "Eric Blake" <eblake@redhat.com>, kvm-devel <kvm@vger.kernel.org>,
"Kyle Evans" <kevans@freebsd.org>,
"QEMU Developers" <qemu-devel@nongnu.org>,
qemu-arm <qemu-arm@nongnu.org>, qemu-ppc <qemu-ppc@nongnu.org>,
"Gerd Hoffmann" <kraxel@redhat.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Philippe Mathieu-Daudé" <philmd@redhat.com>
Subject: Re: [PATCH v2 4/9] bsd-user/syscall: Replace alloca() by g_new()
Date: Thu, 6 May 2021 16:58:47 +0100 [thread overview]
Message-ID: <CAFEAcA-V1DWhsFYuh-y5F2_PbO50KFoCm-XPrcMEYN1V2WHDfA@mail.gmail.com> (raw)
In-Reply-To: <CANCZdfqW0XTa18F+JxuSnhpictWxVJUsu87c=yAwMp6YT60FMg@mail.gmail.com>
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
next prev parent reply other threads:[~2021-05-06 16:00 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-06 13:37 [PATCH v2 0/9] misc: Replace alloca() by g_malloc() Philippe Mathieu-Daudé
2021-05-06 13:37 ` [PATCH v2 1/9] audio/alsaaudio: Replace ALSA alloca() by malloc() equivalent Philippe Mathieu-Daudé
2021-05-06 13:37 ` [PATCH v2 2/9] backends/tpm: Replace qemu_mutex_lock calls with QEMU_LOCK_GUARD Philippe Mathieu-Daudé
2021-05-06 14:46 ` Stefan Berger
2021-05-06 15:07 ` Christophe de Dinechin
2021-05-06 13:37 ` [PATCH v2 3/9] backends/tpm: Replace g_alloca() by g_malloc() Philippe Mathieu-Daudé
2021-05-06 14:46 ` Stefan Berger
2021-05-06 13:37 ` [PATCH v2 4/9] bsd-user/syscall: Replace alloca() by g_new() Philippe Mathieu-Daudé
[not found] ` <CANCZdfoJWEbPFvZ0605riUfnpVRAeC6Feem5_ahC7FUfO71-AA@mail.gmail.com>
2021-05-06 14:20 ` Peter Maydell
[not found] ` <CANCZdfpXjDECHmZq55zP43g32OVhnfjf9W+ERtPMFeDs2MmvXQ@mail.gmail.com>
2021-05-06 14:57 ` Philippe Mathieu-Daudé
2021-05-06 14:25 ` Eric Blake
[not found] ` <CANCZdfqW0XTa18F+JxuSnhpictWxVJUsu87c=yAwMp6YT60FMg@mail.gmail.com>
2021-05-06 15:12 ` Eric Blake
[not found] ` <CANCZdfrcv9ZUcBv7z+z3JPCjy0uzzY07VLmC4dqr5r8ba_QPLw@mail.gmail.com>
2021-05-06 15:42 ` Eric Blake
2021-05-06 16:03 ` Peter Maydell
2021-05-06 15:58 ` Peter Maydell [this message]
2021-05-06 13:37 ` [PATCH v2 5/9] gdbstub: Constify GdbCmdParseEntry Philippe Mathieu-Daudé
2021-05-06 13:37 ` [PATCH v2 6/9] gdbstub: Only call cmd_parse_params() with non-NULL command schema Philippe Mathieu-Daudé
2021-05-06 19:21 ` Alex Bennée
2021-05-06 13:37 ` [PATCH v2 7/9] gdbstub: Replace alloca() + memset(0) by g_new0() Philippe Mathieu-Daudé
2021-05-06 19:22 ` Alex Bennée
2021-05-06 13:37 ` [PATCH v2 8/9] hw/misc/pca9552: Replace g_newa() by g_new() Philippe Mathieu-Daudé
2021-05-06 13:37 ` [PATCH v2 9/9] target/ppc/kvm: Replace alloca() by g_malloc() Philippe Mathieu-Daudé
2021-05-06 14:45 ` Greg Kurz
2021-05-07 1:05 ` David Gibson
[not found] ` <CANCZdfqiHxQoG+g3bq_KL01yWCHUbF5qxJWN=sD37h7UJFMZ7g@mail.gmail.com>
2021-05-06 14:28 ` [PATCH v2 0/9] misc: " Eric Blake
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