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[Qemu-devel] re: PRoot: A Step Forward for QEMU User-Mode

From: Riku Voipio
Subject: [Qemu-devel] re: PRoot: A Step Forward for QEMU User-Mode
Date: Mon, 21 Mar 2011 15:42:56 +0200
User-agent: Mutt/1.5.18 (2008-05-17)


First, thanks for the report!

On Mon, Mar 21, 2011 at 12:09:10PM +0000, Peter Maydell wrote:
> == Talk 11: PRoot: A Step Forward for QEMU User-Mode ==
> STMicroelectronics again, presenting an alternative to the usual
> "chroot plus binfmt_misc" approach for running target binaries
> seamlessly under qemu's linux-user mode.  It's a wrapper around qemu
> which uses ptrace to intercept the syscalls qemu makes to the host; in
> particular it can add the target-directory prefix to all filesystem
> access syscalls, and can turn an attempt to exec "/bin/ls" into an
> exec of "qemu-linux-arm /bin/ls". The advantage over chroot is that
> it's more flexible and doesn't need root access to set up. They didn't
> give figures for how much overhead the syscall interception adds,
> though.

Similar thing is done with scratchbox2, but with LD_PRELOAD instead of
ptrace. LD_PRELOAD for all around the corners (static binaries etc) and
the slowdown is major. ptrace would make at least the former problem
go away.

However, why do it behind Qemu's back anyway? In linux-user we already pass
most filesystem access via path() function. Any needed filesystem redirections
could be done there within Qemu itself.


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