Messages in this thread | | | Date | Wed, 15 Dec 2004 05:27:04 +0100 | From | Andi Kleen <> | Subject | Re: 32-bit syscalls from 64-bit process on x86-64? |
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On Tue, Dec 14, 2004 at 11:01:12PM +0100, Petr Vandrovec wrote: > On 14 Dec 04 at 8:45, Andi Kleen wrote: > > > #define TOLM \ > > > "pushl %%cs\n" \ > > > "pushl $91f\n" \ > > > "ljmpl $0x33,$90f\n" \ > > > > It's useless, there is nothing in the kernel code that checks the > > 32bit segment. > > ??? Processor checks for 32bit/64bit segment. It is impossible to load > upper 32bit of all registers with non-zero value or call 64bit > syscall entry point from 32bit mode. As x86-64 kernel offers 64bit > interface through syscall only, only way how to issue 64bit system call > is using syscall instruction in 64bit code.
Ah sorry. I misread the intention of your code. I thought you wanted to do it the other way round - 32bit syscall from 64bit code. I just wanted to point out that you can do it directly without changing the code segment, as long as you use int $0x80.
From 64bit-from-32bit the lcall is needed agreed. However as a warning it will not work for all calls since a few check a bit in task_struct that says if the process is 32bit or 64bit (rather rare though, most prominent is signal handling)
> > Or are you trying to say that these samples do not work and you cannot > call 64bit entry point from 32bit app, or vice versa? Then I'm afraid > that you are not completely right, as these samples do work...
I haven't ever tried it, but I see no reason it cannot work.
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