Re: [PATCH v9 35/39] kselftest/arm64: Add a GCS test program built with the system libc

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

Mark Brown <broonie@kernel.org> writes:

> There are things like threads which nolibc struggles with which we want
> to add coverage for, and the ABI allows us to test most of these even if
> libc itself does not understand GCS so add a test application built
> using the system libc.
>
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  tools/testing/selftests/arm64/gcs/.gitignore |   1 +
>  tools/testing/selftests/arm64/gcs/Makefile   |   4 +-
>  tools/testing/selftests/arm64/gcs/gcs-util.h |  10 +
>  tools/testing/selftests/arm64/gcs/libc-gcs.c | 736 +++++++++++++++++++++++++++
>  4 files changed, 750 insertions(+), 1 deletion(-)

In my FVP VM, this test gets a GCS SIGSEGV before running the first test:

$ sudo ./run_kselftest.sh -t arm64:libc-gcs
[sudo] password for bauermann:
TAP version 13
1..1
# timeout set to 45
# selftests: arm64: libc-gcs
# TAP version 13
# 1..118
# # Starting 118 tests from 32 test cases.
# #  RUN           global.can_call_function ...
# Segmentation fault
not ok 1 selftests: arm64: libc-gcs # exit=139
$ 

It happens when returning from the syscall() glibc function that does
the clone3 syscall in kselftest_harness.h:

$ /var/tmp/gdb-gcs/bin/gdb -q arm64/libc-gcs
Reading symbols from arm64/libc-gcs...
(gdb) r
Starting program: /var/tmp/selftests-v9/arm64/libc-gcs
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".
TAP version 13
1..118
# Starting 118 tests from 32 test cases.
#  RUN           global.can_call_function ...
[Detaching after vfork from child process 823]

Program received signal SIGSEGV, Segmentation fault
Guarded Control Stack error.
syscall () at ../sysdeps/unix/sysv/linux/aarch64/syscall.S:41 [GCS error]
warning: 41     ../sysdeps/unix/sysv/linux/aarch64/syscall.S: No such file or directory
(gdb) p $_siginfo.si_signo
$1 = 11
(gdb) p $_siginfo.si_code
$2 = 10
(gdb) p $_siginfo._sifields._sigfault.si_addr
$3 = (void *) 0xfffff7ed96b0 <syscall+48>
(gdb) disassemble
Dump of assembler code for function syscall:
   0x0000fffff7ed9680 <+0>:     nop
   0x0000fffff7ed9684 <+4>:     mov     w8, w0
   0x0000fffff7ed9688 <+8>:     mov     x0, x1
   0x0000fffff7ed968c <+12>:    mov     x1, x2
   0x0000fffff7ed9690 <+16>:    mov     x2, x3
   0x0000fffff7ed9694 <+20>:    mov     x3, x4
   0x0000fffff7ed9698 <+24>:    mov     x4, x5
   0x0000fffff7ed969c <+28>:    mov     x5, x6
   0x0000fffff7ed96a0 <+32>:    mov     x6, x7
   0x0000fffff7ed96a4 <+36>:    svc     #0x0
   0x0000fffff7ed96a8 <+40>:    cmn     x0, #0xfff
   0x0000fffff7ed96ac <+44>:    b.cs    0xfffff7ed96b4 <syscall+52>  // b.hs, b.nlast
=> 0x0000fffff7ed96b0 <+48>:    ret
   0x0000fffff7ed96b4 <+52>:    b       0xfffff7e18660 <__GI___syscall_error>
   0x0000fffff7ed96b8 <+56>:    b       0xfffff7e18660 <__GI___syscall_error>
End of assembler dump.
(gdb) bt
#0  syscall () at ../sysdeps/unix/sysv/linux/aarch64/syscall.S:41 [GCS error]
#1  0x0000aaaaaaaa4acc in clone3_vfork ()
    at /home/bauermann/src/linux/tools/testing/selftests/kselftest_harness.h:93
#2  __run_test (f=f@entry=0xaaaaaaac0b88 <_fixture_global>, variant=variant@entry=0xffffffffee00,
    t=t@entry=0xaaaaaaac0018 <_can_call_function_object>)
    at /home/bauermann/src/linux/tools/testing/selftests/kselftest_harness.h:1239 [GCS error]
#3  0x0000aaaaaaaa2c40 in test_harness_run (argv=0xfffffffff008, argc=1)
    at /home/bauermann/src/linux/tools/testing/selftests/kselftest_harness.h:1314
#4  main (argc=1, argv=0xfffffffff008) at libc-gcs.c:735 [GCS error]
(gdb) 

And indeed, the svc call in the disassemble above corrupts the GCS.
This is the GCS and lr values right before the svc call:

(gdb) x/i $pc
=> 0xfffff7ed96a4 <syscall+36>: svc     #0x0
(gdb) p/x $lr
$3 = 0xaaaaaaaa4acc
(gdb) p/x $gcspr
$4 = 0xfffff7bfffe8
(gdb) x/g $gcspr
0xfffff7bfffe8: 0x0000aaaaaaaa4acc

So far so good, the tip of the GCS matches $lr. But then:

(gdb) stepi
[Detaching after vfork from child process 2491]
39      in ../sysdeps/unix/sysv/linux/aarch64/syscall.S
(gdb) x/i $pc
=> 0xfffff7ed96a8 <syscall+40>: cmn     x0, #0xfff
(gdb) p/x $gcspr
$5 = 0xfffff7bfffe8
(gdb) x/g $gcspr
0xfffff7bfffe8: 0x0000aaaaaaaa4c04
(gdb) p/x $lr
$6 = 0xaaaaaaaa4acc

So, right after svc returns, the tip of the GCS is corrupted and will
cause the GCS error.

-- 
Thiago