RE: in_compat_syscall() on x86

[David Laight]

Copy x86@xxxxxxxxxx

> -----Original Message-----
> From: David Laight <David.Laight@xxxxxxxxxx>
> Sent: 04 January 2021 12:17
> To: Al Viro <viro@xxxxxxxxxxxxxxxxxx>; Christoph Hellwig <hch@xxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> 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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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)