Re: selftests: sigaltstack: sas # exit=1 - # Bail out! SP is not on sigaltstack - on clang build

[Andy Lutomirski]

On Tue, Mar 7, 2023 at 7:14 PM Naresh Kamboju <naresh.kamboju@xxxxxxxxxx> wrote:
>
> + LLVM

The offending code seems to be:

#if __s390x__
        register unsigned long sp asm("%15");
#else
        register unsigned long sp asm("sp");
#endif

        if (sp < (unsigned long)sstack ||
                        sp >= (unsigned long)sstack + stack_size) {
                ksft_exit_fail_msg("SP is not on sigaltstack\n");
        }

Is that actually expected to work?  asm("sp") is a horrible hack.  I
would, maybe naively, expect a compiler to analyze this code, think
"sp is unconditionally uninitialized", and treat the comparison as
always-UB and thus generate whatever code seems convenient.

--Andy

>
> On Wed, 8 Mar 2023 at 00:58, Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
> >
> > On 3/6/2023 10:57 PM, Naresh Kamboju wrote:
> > > kselftest: sigaltstack built with clang-16 getting failed but passed with
> > > gcc-12 build. Please find more details about test logs on clang-16 and
> > > gcc-12 and steps to reproduce locally on your machine by using tuxrun.
> > >
> > > Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> > >
> > > Test log:
> > > ----------
> > >
> > > Linux version 6.3.0-rc1-next-20230307 (tuxmake@tuxmake) (Debian clang
> > > version 16.0.0 (++20230228093516+60692a66ced6-1~exp1~20230228093525.41),
> > > Debian LLD 16.0.0) #1 SMP PREEMPT @1678159722
> > > ...
> > > kselftest: Running tests in sigaltstack
> > > TAP version 13
> > > 1..1
> > > # selftests: sigaltstack: sas
> > > # # [NOTE] the stack size is 21104
> > > # TAP version 13
> > > # 1..3
> > > # ok 1 Initial sigaltstack state was SS_DISABLE
> > > # Bail out! SP is not on sigaltstack
> > > # # Planned tests != run tests (3 != 1)
> > > # # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> > > not ok 1 selftests: sigaltstack: sas # exit=1
> > <snip>
> >
> > > Linux version 6.3.0-rc1-next-20230307 (tuxmake@tuxmake)
> > > (aarch64-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils
> > > for Debian) 2.40) #1 SMP PREEMPT @1678159736
> > > ...
> > > kselftest: Running tests in sigaltstack
> > > TAP version 13
> > > 1..1
> > > # selftests: sigaltstack: sas
> > > # # [NOTE] the stack size is 50080
> > > # TAP version 13
> > > # 1..3
> > > # ok 1 Initial sigaltstack state was SS_DISABLE
> > > # # [RUN] signal USR1
> > > # ok 2 sigaltstack is disabled in sighandler
> > > # # [RUN] switched to user ctx
> > > # # [RUN] signal USR2
> > > # # [OK] Stack preserved
> > > # ok 3 sigaltstack is still SS_AUTODISARM after signal
> > > # # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0
> > > ok 1 selftests: sigaltstack: sas
> >
> > At glance, the log shows the altstack size difference between LLVM and GCC.
> >
> > But, when I tried with the LLVM that I have,
> >
> >      $ clang --version
> >      clang version 13.0.0 ...
> >
> > it failed only with this compiler:
> >
> >      $ rm sas;clang -o sas sas.c;./sas
> >      # [NOTE]        the stack size is 8192
> >      TAP version 13
> >      1..3
> >      ok 1 Initial sigaltstack state was SS_DISABLE
> >      Bail out! SP is not on sigaltstack
> >      # Planned tests != run tests (3 != 1)
> >      # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> >
> >      $ rm sas;gcc -o sas sas.c;./sas
> >      # [NOTE]        the stack size is 8192
> >      TAP version 13
> >      1..3
> >      ok 1 Initial sigaltstack state was SS_DISABLE
> >      # [RUN] signal USR1
> >      ok 2 sigaltstack is disabled in sighandler
> >      # [RUN] switched to user ctx
> >      # [RUN] signal USR2
> >      # [OK]  Stack preserved
> >      ok 3 sigaltstack is still SS_AUTODISARM after signal
> >      # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0
> >
> > The same is true with some old versions -- e.g. the one that came with
> > commit 0c49ad415512 ("tools/testing/selftests/sigaltstack/sas.c: improve
> > output of sigaltstack testcase"):
> >
> >      $ rm sas;clang -o sas sas.c;./sas
> >      [OK]    Initial sigaltstack state was SS_DISABLE
> >      [FAIL]  SP is not on sigaltstack
> >
> >      $ rm sas;gcc -o sas sas.c;./sas
> >      [OK]    Initial sigaltstack state was SS_DISABLE
> >      [RUN]   signal USR1
> >      [OK]    sigaltstack is disabled in sighandler
> >      [RUN]   switched to user ctx
> >      [RUN]   signal USR2
> >      [OK]    Stack preserved
> >      [OK]    sigaltstack is still SS_AUTODISARM after signal
> >      [OK]    Test passed
> >
> > So, this test failure appears to have been there for a while. I think
> > the LLVM folks need to take a look at it.
> >
> > Thanks,
> > Chang