The regfile provided to SA_SIGINFO signal handler as ucontext was off by
one due to pt_regs gutter cleanups in 2013.
Before handling signal, user pt_regs are copied onto user_regs_struct and copied
back later. Both structs are binary compatible. This was all fine until
commit 2fa919045b (ARC: pt_regs update #2) which removed the empty stack slot
at top of pt_regs (corresponding to first pad) and made the corresponding
fixup in struct user_regs_struct (the pad in there was moved out of
@scratch - not removed altogether as it is part of ptrace ABI)
struct user_regs_struct {
+ long pad;
struct {
- long pad;
long bta, lp_start, lp_end,....
} scratch;
...
}
This meant that now user_regs_struct was off by 1 reg w.r.t pt_regs and
signal code needs to user_regs_struct.scratch to reflect it as pt_regs,
which is what this commit does.
This problem was hidden for 2 years, because both save/restore, despite
using wrong location, were using the same location. Only an interim
inspection (reproducer below) exposed the issue.
void handle_segv(int signo, siginfo_t *info, void *context)
{
ucontext_t *uc = context;
struct user_regs_struct *regs = &(uc->uc_mcontext.regs);
printf("regs %x %x\n", <=== prints 7 8 (vs. 8 9)
regs->scratch.r8, regs->scratch.r9);
}
int main()
{
struct sigaction sa;
sa.sa_sigaction = handle_segv;
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
sigaction(SIGSEGV, &sa, NULL);
asm volatile(
"mov r7, 7 \n"
"mov r8, 8 \n"
"mov r9, 9 \n"
"mov r10, 10 \n"
:::"r7","r8","r9","r10");
*((unsigned int*)0x10) = 0;
}
Fixes: 2fa919045b "ARC: pt_regs update #2: Remove unused gutter at start of pt_regs"
CC: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
6914e1e3f6