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From: Laurent Vivier <laurent@vivier.eu>
To: Richard Henderson <richard.henderson@linaro.org>, qemu-devel@nongnu.org
Cc: pbonzini@redhat.com, riku.voipio@iki.fi, peter.maydell@linaro.org
Subject: Re: [PATCH 0/3] linux-user: Implement x86_64 vsyscalls
Date: Wed, 15 Jan 2020 11:14:35 +0100	[thread overview]
Message-ID: <7f8c782f-6ee2-bc7c-db94-4b0784ed58eb@vivier.eu> (raw)
In-Reply-To: <20200114210921.11216-1-richard.henderson@linaro.org>

Le 14/01/2020 à 22:09, Richard Henderson a écrit :
> The x86_64 abi has a legacy vsyscall page.  The kernel folk
> have been trying to deprecate this since at least v3.1, but
> 
> (1) We don't implement the vdso that replaces vsyscalls,
> (2) As of v5.5, the vsyscall page is still enabled by default.
> 
> This lack is affecting Peter's linux-user testing.
> 
> The dependency is not obvious because Peter is running the tests
> on x86_64, so the host is providing a vsyscall page to qemu.
> 
> Because of how user-only memory operations are handled, with no
> validation of guest vs host pages, so long as qemu chooses to
> run with guest_base == 0, the guest may Just So Happen to read
> the host's vsyscall page.
> 
> Complicating this, new OS releases may use a kernel configured
> with CONFIG_LEGACY_VSYSCALL_XONLY=y, which means the the vsyscall
> page cannot be read, only executed.  Which means that the guest
> then cannot read the host vsyscall page during translation and
> will SIGSEGV.
> 
> Exactly which of these many variables is affecting Peter's testing
> with Ubuntu 18.04 of my TCG merge, I'm not exactly sure.  I suspect
> that it is the change to drop the textseg_addr adjustment to user-only
> static binaries.  IIRC bionic does not support -static-pie, which is
> the preferred replacement.  This could mean that the host and guest
> binaries overlap, which leads to guest_base != 0.
> 
> I vaguely remember someone (Paolo?) implementing something like
> this many years ago, but clearly it never got merged.
> 
> In any case, this emulation has been missing for too long.
> 
> 
> r~
> 
> 
> Richard Henderson (3):
>   target/i386: Renumber EXCP_SYSCALL
>   linux-user/i386: Split out gen_signal
>   linux-user/i386: Emulate x86_64 vsyscalls
> 
>  target/i386/cpu.h          |   6 +-
>  linux-user/i386/cpu_loop.c | 197 ++++++++++++++++++++++++++-----------
>  target/i386/translate.c    |  16 ++-
>  3 files changed, 155 insertions(+), 64 deletions(-)
> 

Thank you Richard.

I'll take this series through the linux-user branch except if you prefer
to do the pull request yourself via another branch (x86 tcg?).

Thanks,
Laurent


  parent reply	other threads:[~2020-01-15 10:15 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 21:09 [PATCH 0/3] linux-user: Implement x86_64 vsyscalls Richard Henderson
2020-01-14 21:09 ` [PATCH 1/3] target/i386: Renumber EXCP_SYSCALL Richard Henderson
2020-01-15  7:22   ` Philippe Mathieu-Daudé
2020-01-15  9:55   ` Alex Bennée
2020-01-14 21:09 ` [PATCH 2/3] linux-user/i386: Split out gen_signal Richard Henderson
2020-01-15  7:22   ` Philippe Mathieu-Daudé
2020-01-15  9:58   ` Alex Bennée
2020-01-14 21:09 ` [PATCH 3/3] linux-user/i386: Emulate x86_64 vsyscalls Richard Henderson
2020-01-16 10:51   ` Alex Bennée
2020-01-16 16:26   ` Alex Bennée
2020-01-16 18:19     ` Richard Henderson
2020-01-16 18:22       ` Richard Henderson
2020-01-16 20:15       ` Alex Bennée
2020-01-14 23:35 ` [PATCH 0/3] linux-user: Implement " Paolo Bonzini
2020-01-15  7:01 ` Laurent Desnogues
2020-01-15 10:14 ` Laurent Vivier [this message]
2020-01-15 17:28   ` Richard Henderson
2020-01-16 14:05 ` Alex Bennée
2020-01-16 19:37   ` Richard Henderson
2020-01-16 14:30 ` Alex Bennée
2020-01-16 18:31   ` Richard Henderson

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