Re: [uml-devel] Bad handling of invalif systemcalls

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

On Thursday 21 October 2004 17:25, Bodo Stroesser wrote:
> If a process in UML does a systemcall with the systemcall number
> being less than 0, in TT-mode and SKAS-mode using SYSEMU, the
> process is killed by an SIGTRAP instead of simply returning -ENOSYS.
> In SKAS-mode without SYSEMU, UML even crashes, no matter if SYSEMU
> is unsupported by the host or switched off in UML:

>      Kernel panic - not syncing: handle_trap - failed to wait at end of
> syscall, errno = 4, status = 2943

> The reason is, that UML can't distinguish between a debugger trap
> and an systemcall interception. Currently, it checks the systemcall
> number. If it is less than 0, it assumes the event to be a debugger
> trap. It would be better to assume a debugger trap only, if the
> syscall number is -1 (which it is guaranteed to be in case of a
> debugger event), but even then UML wouldn't be safe. Syscalls with
> syscall number -1 still would be a problem!

> AFAICS, the only solution for this is using the PTRACE_O_TRACESYSGOOD
> option. This option seems to be specific for linux, but the problem
> maybe is specific for linux, too.

I have been thinking to using SYSGOOD, but without finding a good reason for 
that... so thanks for this.

The first remark (the only I have for now) is that you should replace "SIGTRAP 
+ 0x80" with "SIGTRAP | 0x80". It should not make a difference in this 
particular case, but it's a style issue, which exists because the second way 
is more robust for setting bits: think about (SIGTRAP & 0x80) == 0x80 and 
using the + 0x80: it clears the bit it should set and set another one.

Remark no. 2: could you, please, in next patches, try to add -p to diff flags 
to improve readability? That makes clear which function is being changed, so 
the patch becomes more readable.
> So, here attached are three patches. The first adds a check for
> availability and function of
>      ptrace(PTRACE_SETOPTIONS,,,PTRACE_O_TRACESYSGOOD) to the normal
> ptrace checks.
>
> The second implements the usage of the option in SKAS-mode.

> The third does the same for TT-mode.

> For the third patch I'm quite anxious, that there could go something
> wrong when using the debugger. I don't understand much about this.
> Maybe someone else could look into this?
I'll give a look when I have the needed time. However, could you explain what 
makes you worry in detail?
> Regards
> Bodo

-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729


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