linux-kernel - in_compat_syscall() on x86

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Date:   Mon, 4 Jan 2021 12:16:56 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: in_compat_syscall() on x86

On x86 in_compat_syscall() is defined as:
    in_ia32_syscall() || in_x32_syscall()

Now in_ia32_syscall() is a simple check of the TS_COMPAT flag.
However in_x32_syscall() is a horrid beast that has to indirect
through to the original %eax value (ie the syscall number) and
check for a bit there.

So on a kernel with x32 support (probably most distro kernels)
the in_compat_syscall() check is rather more expensive than
one might expect.

It would be muck better if both checks could be done together.
I think this would require the syscall entry code to set a
value in both the 64bit and x32 entry paths.
(Can a process make both 64bit and x32 system calls?)

To do this sensible (probably) requires a byte be allocated
to hold the syscall type - rather than using flag bits
in the 'status' field.

Apart from the syscall entry, the exec code seems to change
the syscall type to that of the binary being executed.
I didn't spot anything else that changes the fields.

But I failed to find the full list of allocated bits for
the 'status' field.

	David

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