cvs commit: src/usr.bin/su su.c

Maxim Sobolev sobomax at
Tue Oct 24 09:48:25 UTC 2006

P.S. Perhaps my mistake is that I have blindly assumed that attempt to 
invoke unimplemented syscall on native platform (i.e. ia32 binary 
running on ia32 kernel) results in the same error condition as attempt 
to invoke unimplemented syscall in binary compatibility mode (i.e. ia32 
binary on amd64 kernel). If it is not, then the binary compatibility 
layer should be fixed instead to match behavior of the native mode.

Maxim Sobolev wrote:
> OK, in this particular case I am trying to run su(8) binary compiled for 
> FreeBSD/ia32 on FreeBSD/amd64 system (FreeBSD 6.2 but this doesn't 
> really make any difference since the code is the same).
> Since all audit syscalls in freebsd32 emulation layer are redirected to 
> nosys() any attempt to invoke such syscall results in both ENOSYS errno 
> *and* SIGSYS signal delivered to the process in question. The latter 
> kills the process without giving it any chance to handle ENOSYS.
> -Maxim
> Robert Watson wrote:
>> On Tue, 24 Oct 2006, Maxim Sobolev wrote:
>>> sobomax     2006-10-24 08:18:10 UTC
>>>  FreeBSD src repository
>>>  Modified files:
>>>    usr.bin/su           su.c
>>>  Log:
>>>  Ignore SIGSYS when BSM is compiled in. Otherwise, attempt to invoke 
>>> su on
>>>  system that don't have audit framefork compiled into kernel or ia32 
>>> binary
>>>  on amd64 system will result in SIGSYS. There is one place in su.c 
>>> itself
>>>  where it tries to check for errno != ENOSYS, but it has been a nop 
>>> since su
>>>  does not catch SIGSYS anyway. There are few other places in libbsm,
>>>  where attempt to invoke audit syscal would result in SIGSYS if no audit
>>>  support is present in the kernel, so that the only reliable method for
>>>  now is to disable SIGSYS completely in the case when BSM is compiled 
>>> in.
>>>  In the long run, both direct invocation of audit-related syscalls and
>>>  libbsm should be made more intellegent to handle the case when BSM 
>>> is not
>>>  compiled into the kernel gracefully.
>>>  MFC after: 3 days
>>>             (provided re@ approval)
>> I have to admit being a bit puzzled by your commit.  Since you didn't 
>> bother to contact me with a bug report before committing this presumed 
>> fix, I can only assume that you are entirely certain that what you're 
>> describing is the case.  However, since it appears not to be the case 
>> on thousands of deployed HEAD and RELENG_6 machines, I have to wonder 
>> whether you're hitting an edge case that needs further debugging and 
>> discussion.
>> Alternatively, you booted a really old kernel with a relatively new 
>> user space.  And by really old, I mean RELENG_6 in March of this year, 
>> or HEAD in February.  How did you manage to log in, as login(1) also 
>> invokes the same system calls unconditionally that su(1) calls?  
>> Returning ENOSYS from a system call stub should not (and in my 
>> experience, does not) generate SIGSYS.  There are three common cases 
>> for unimplemented system calls:
>> (1) The kernel doesn't recognize the system call.  Return ENOSYS + 
>> SIGSYS.  If
>>     a binary handles SIGSYS, it gets the ENOSYS return, otherwise, it 
>> exits.
>>     This occurs if the system call isn't listed in the system call 
>> table, or
>>     if it's listed but unimplemented.  This is what happens if you 
>> invoke a
>>     random system call number, or a new system call on a really old 
>> kernel.
>> (2) The kernel recognizes, but does not implement the system call, and 
>> the
>>     system call is one where error handling is possible.  This occurs 
>> when a
>>     system call stub function returns ENOSYS.  For example, the audit 
>> code
>>     contains conditionally compiled system call code that returns 
>> ENOSYS if
>>     AUDIT isn't in the kernel.  This also occurs when compatibility 
>> stubs are
>>     inserted to allow forward compatibility.
>> (3) The kernel recognizes and implements the system call, but the 
>> underlying
>>     object doesn't implement the call.  Return EOPNOTSUPP, ENOTSUP, 
>>     etc.  Typical examples are VFS system calls where the file system 
>> doesn't
>>     implement the operation, such as extended attributes, hard links, 
>> etc.
>> The audit subsystem uses the second approach so that userland binaries 
>> can detect that audit is not available and handle it properly 
>> (typically by not configuring audit and generating audit events).  
>> Notice that we merged audit system call stubs to RELENG_6 for 
>> 6.1-RELEASE so that the upgrade path to an audit-enabled userland in 
>> 6.2-RELEASE would be un-bumpy if the user didn't properly follow 
>> proper upgrade instructions (kernel before user space).
>> So: could you confirm that what you're observing is happening and 
>> provide specific specific debugging information?  For example, your 
>> commit message hand-waves at 32-bit binaries on amd64 -- perhaps there 
>> is a bug in the handling of ENOSYS in the compat/freebsd32 system call 
>> path.  A casual reading of the system call code suggests that this is 
>> not the case, but I don't think this is a case I have specifically 
>> tested.  If you're running a very old kernel (pre-February/March) with 
>> a new user space, understand that that is not a supported 
>> configuration that will break for other more subtle reasons, and 
>> please back out this commit.
>> Thanks,
>> Robert N M Watson
>> Computer Laboratory
>> University of Cambridge

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