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From: "Amanieu d'Antras" <amanieu@gmail.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Ryan Houdek <Houdek.Ryan@fex-emu.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	 Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	 Steven Price <steven.price@arm.com>,
	David Laight <David.Laight@aculab.com>,
	 Mark Brown <broonie@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls
Date: Wed, 19 May 2021 00:51:00 +0100	[thread overview]
Message-ID: <CA+y5pbRiXAF=gobC9sqJmvjVAmihA=O7xcSTkA1f8=QsnZzoEg@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a0=iSUBu5GnuWoxEjB0Hpd3iHeVwe2Njfj6x64hoJA5oA@mail.gmail.com>

On Tue, May 18, 2021 at 2:03 PM Arnd Bergmann <arnd@kernel.org> wrote:
> I'm still undecided about this approach. It is an easy way to expose the 32-bit
> ABIs, it mostly copies what x86-64 already does with 32-bit syscalls and
> it doesn't expose a lot of attack surface that isn't already exposed to normal
> 32-bit tasks running compat mode.
>
> On the other hand, exposing the entire aarch32 syscall set seems both
> too broad and not broad enough: Half of the system calls behave the
> exact same way in native and compat mode, so they wouldn't need to
> be exposed like this, a lot of others are trivially emulated in user space
> by calling the native versions. The syscalls that are actually hard to do
> such as ioctl() or the signal handling will work for aarch32 emulation, but
> they are still insufficient to correctly emulate other 32-bit architectures
> that have a slightly different ABI. This means the interface is a fairly good
> fit for Tango, but much less so for FEX.
>
> It's also worth pointing out that this approach has a few things in common
> with Yury's ilp32 tree at https://github.com/norov/linux/tree/ilp32-5.2
> Unlike the x86 x32 mode, that port however does not allow calling compat
> syscalls from normal 64-bit tasks but rather keys the syscall entry point
> off the executable format., which wouldn't work here. It also uses the
> asm-generic system call numbers instead of the arm32 syscall numbers.
>
> I assume you have already considered or tried the alternative approach of
> only adding a minimal set of syscalls that are needed for the emulation.
> Having a way to limit the address space for mmap() and similar
> system calls sounds like a generally useful addition, and having an
> extended variant of ioctl() that lets you pick the target ABI (arm32, x86-32,
> ...) on supported drivers would probably be better for FEX. Can you
> explain the tradeoffs that led you towards duplicating the syscall
> entry points instead?

Tango needs the entire compat ABI to be exposed to support seccomp for
translated AArch32 processes. Here's how this works:

1. When a translated process installs a seccomp filter, Tango injects
a prefix into the seccomp program which effectively does:
    if (arch == AUDIT_ARCH_AARCH64) {
        // 64-bit syscalls used by Tango for internal operations
        if (syscall_in_tango_whitelist(nr))
            return SECCOMP_RET_ALLOW;
    }
    // continue to user-supplied seccomp program

2. When Tango performs a 32-bit syscall on behalf of the translated
process, the seccomp filter will see a syscall with AUDIT_ARCH_ARM and
the compat syscall number. This allows the user-supplied seccomp
filter to behave exactly as if it was running in a native AArch32
process.

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  parent reply	other threads:[~2021-05-18 23:53 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-18  9:06 [RESEND PATCH v4 0/8] " Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 1/8] mm: Add arch_get_mmap_base_topdown macro Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 2/8] hugetlbfs: Use arch_get_mmap_* macros Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 3/8] mm: Support mmap_compat_base with the generic layout Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 4/8] arm64: Separate in_compat_syscall from is_compat_task Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 5/8] arm64: mm: Use HAVE_ARCH_COMPAT_MMAP_BASES Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 6/8] arm64: Add a compat syscall flag to thread_info Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 7/8] arm64: Forbid calling compat sigreturn from 64-bit tasks Amanieu d'Antras
2021-05-18  9:06 ` [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls Amanieu d'Antras
2021-05-18 13:02   ` Arnd Bergmann
2021-05-18 20:26     ` David Laight
2021-05-18 22:41       ` Ryan Houdek
2021-05-18 23:51     ` Amanieu d'Antras [this message]
2021-05-19 15:30       ` Steven Price
2021-05-19 16:14         ` Amanieu d'Antras
2021-05-21  8:51           ` Steven Price
2021-05-21 19:18             ` Amanieu d'Antras
2021-05-24 11:20               ` Steven Price
2021-05-24 12:38                 ` David Laight
2021-05-18 23:52     ` Ryan Houdek

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