https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66631
Bug ID: 66631
Summary: inability to clobber segment regs makes tls
problematic
Product: gcc
Version: 5.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: stsp at users dot sourceforge.net
Target Milestone: ---
Hello.
On x86 (32/64) gcc uses FS register for TLS.
When writing a signal handler, a special care
must be taken: FS register must be restored by
hands, together with all other segment registers,
because linux unfortunately does not do that
(except cs and ds).
Then signal handler can access the thread-local
data safely... except that it can't.
After calling the function that restores the
segment registers, I need to specify that FS was
clobbered, which is currently not possible with
the inline asm.
As a result, the following code is generated:
0x00000000004438c0 <+0>: push %r14
0x00000000004438c2 <+2>: mov %edi,%r14d
0x00000000004438c5 <+5>: push %r13
0x00000000004438c7 <+7>: mov %rdx,%r13
0x00000000004438ca <+10>: push %r12
0x00000000004438cc <+12>: lea 0x28(%rdx),%r12
0x00000000004438d0 <+16>: push %rbp
0x00000000004438d1 <+17>: mov %r12,%rdi
0x00000000004438d4 <+20>: mov %fs:0x0,%rbp
0x00000000004438dd <+29>: push %rbx
0x00000000004438de <+30>: add $0xffffffffffffff80,%rsp
0x00000000004438e2 <+34>: callq 0x4441b0 <init_handler>
0x00000000004438e7 <+39>: mov 0x35070a(%rip),%rbx # 0x793ff8
0x00000000004438ee <+46>: mov 0x0(%rbp,%rbx,1),%eax <-- STACK FAULT
init_handler() is a function that restore the segregs -
it is called first in a function:
---
{
init_handler(scp);
// asm volatile("":::"fs");
fault_cnt++;
---
fault_cnt is a thread-local var:
extern volatile __thread int fault_cnt;
As you can see, gcc doesn't move the access to
fault_cnt below init_handler(), but, unfortunately,
it moved the address calculation. To prevent this,
I need to clobber the FS, but it doesn't seem to be
possible.
Please let me know if there are any work-arounds to that.
It seems to be a bug that gcc does not allow clobbering
the segregs that it use.
