On 01/28, Oleg Nesterov wrote:
>
> On 01/28, Andrey Wagin wrote:
> >
> > We use hardware breakpoints in CRIU and we found that sometimes we set
> > a break-point, but a process doesn't stop on it.
>
> reproduced, and this certainly looks like kvm bug to me.
>
> > The reproducer uses a different break-point address if it is executed
> > with arguments than when it executed without arguments.
>
> IOW, multiple processes running in parallel use the same debug register db0
> but different address. And it seems that set_debugreg(address, 0) sometime
> doesn't work in the guest kernel.
>
> I think I verified the following:
>
> - debug registers look always correct as it seen by the guest.
> I used get_debugreg() to dump them after the task misses bp.
>
> - do_debug() was not called in this case.
>
> - finally, it seems that the host has the wrong value in db0
> set by another process.
>
> I modified your test-case so that child2() calls child() when
> it detects the missed bp, and this does trigger do_debug/etc
> while it should not.
See another test-case below.
I am running "./bp 0 1" on the host and "./bp 14 15" under QEMU, this immediately
leads to
ERR!! hit wrong bp 0 != 14
ERR!! hit wrong bp 0 != 14
ERR!! hit wrong bp 0 != 14
ERR!! hit wrong bp 1 != 14
...
Oleg.
-------------------------------------------------------------------------------
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <asm/debugreg.h>
#include <assert.h>
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
unsigned long encode_dr7(int drnum, int enable, unsigned int type, unsigned int len)
{
unsigned long dr7;
dr7 = ((len | type) & 0xf)
<< (DR_CONTROL_SHIFT + drnum * DR_CONTROL_SIZE);
if (enable)
dr7 |= (DR_GLOBAL_ENABLE << (drnum * DR_ENABLE_SIZE));
return dr7;
}
int write_dr(int pid, int dr, unsigned long val)
{
return ptrace(PTRACE_POKEUSER, pid,
offsetof (struct user, u_debugreg[dr]),
val);
}
void set_bp(pid_t pid, void *addr)
{
unsigned long dr7;
assert(write_dr(pid, 0, (long)addr) == 0);
dr7 = encode_dr7(0, 1, DR_RW_EXECUTE, DR_LEN_1);
assert(write_dr(pid, 7, dr7) == 0);
}
void *get_rip(int pid)
{
return (void*)ptrace(PTRACE_PEEKUSER, pid,
offsetof(struct user, regs.rip), 0);
}
void test(int nr)
{
void *bp_addr = &&label + nr, *bp_hit;
int pid;
printf("test bp %d\n", nr);
assert(nr < 16); // see 16 asm nops below
pid = fork();
if (!pid) {
assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
kill(getpid(), SIGSTOP);
for (;;) {
label: asm (
"nop; nop; nop; nop;"
"nop; nop; nop; nop;"
"nop; nop; nop; nop;"
"nop; nop; nop; nop;"
);
}
}
assert(pid == wait(NULL));
set_bp(pid, bp_addr);
for (;;) {
assert(ptrace(PTRACE_CONT, pid, 0, 0) == 0);
assert(pid == wait(NULL));
bp_hit = get_rip(pid);
if (bp_hit != bp_addr)
fprintf(stderr, "ERR!! hit wrong bp %ld != %d\n",
bp_hit - &&label, nr);
}
}
int main(int argc, const char *argv[])
{
while (--argc) {
int nr = atoi(*++argv);
if (!fork())
test(nr);
}
while (wait(NULL) > 0)
;
return 0;
}
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