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, EINVAL,
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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