Messages in this thread | | | From | David Laight <> | Subject | in_compat_syscall() on x86 | Date | Mon, 4 Jan 2021 12:16:56 +0000 |
| |
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
- Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
| |