selftests/x86: Support Atom for syscall_arg_fault test [Linux 5.2]

selftests/x86: Support Atom for syscall_arg_fault test [Linux 5.2]

This Linux kernel change "selftests/x86: Support Atom for syscall_arg_fault test" is included in the Linux 5.2 release. This change is authored by Tong Bo <bo.tong [at]> on Fri Apr 19 15:10:55 2019 +0800. The commit for this change in Linux stable tree is a20d452 (patch).

selftests/x86: Support Atom for syscall_arg_fault test

Atom-based CPUs trigger stack fault when invoke 32-bit SYSENTER instruction
with invalid register values. So we also need SIGBUS handling in this case.

Following is assembly when the fault exception happens.

(gdb) disassemble $eip
Dump of assembler code for function __kernel_vsyscall:
   0xf7fd8fe0 <+0>:     push   %ecx
   0xf7fd8fe1 <+1>:     push   %edx
   0xf7fd8fe2 <+2>:     push   %ebp
   0xf7fd8fe3 <+3>:     mov    %esp,%ebp
   0xf7fd8fe5 <+5>:     sysenter
   0xf7fd8fe7 <+7>:     int    $0x80
=> 0xf7fd8fe9 <+9>:     pop    %ebp
   0xf7fd8fea <+10>:    pop    %edx
   0xf7fd8feb <+11>:    pop    %ecx
   0xf7fd8fec <+12>:    ret
End of assembler dump.

According to Intel SDM, this could also be a Stack Segment Fault(#SS, 12),
except a normal Page Fault(#PF, 14). Especially, in section 6.9 of Vol.3A,
both stack and page faults are within the 10th(lowest priority) class, and
as it said, "exceptions within each class are implementation-dependent and
may vary from processor to processor". It's expected for processors like
Intel Atom to trigger stack fault(SIGBUS), while we get page fault(SIGSEGV)
from common Core processors.

Signed-off-by: Tong Bo <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

There are 10 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 tools/testing/selftests/x86/syscall_arg_fault.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/x86/syscall_arg_fault.c b/tools/testing/selftests/x86/syscall_arg_fault.c
index 7db4fc9..d2548401 100644
--- a/tools/testing/selftests/x86/syscall_arg_fault.c
+++ b/tools/testing/selftests/x86/syscall_arg_fault.c
@@ -43,7 +43,7 @@ static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),

 static volatile sig_atomic_t n_errs;

-static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
+static void sigsegv_or_sigbus(int sig, siginfo_t *info, void *ctx_void)
    ucontext_t *ctx = (ucontext_t*)ctx_void;

@@ -73,7 +73,13 @@ int main()
    if (sigaltstack(&stack, NULL) != 0)
        err(1, "sigaltstack");

-   sethandler(SIGSEGV, sigsegv, SA_ONSTACK);
+   sethandler(SIGSEGV, sigsegv_or_sigbus, SA_ONSTACK);
+   /*
+    * The actual exception can vary.  On Atom CPUs, we get #SS
+    * instead of #PF when the vDSO fails to access the stack when
+    * ESP is too close to 2^32, and #SS causes SIGBUS.
+    */
+   sethandler(SIGBUS, sigsegv_or_sigbus, SA_ONSTACK);
    sethandler(SIGILL, sigill, SA_ONSTACK);


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