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FatalSignalHandler: use mmap() to create the alternate stack

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So we can mark the bottom page as inaccessible, thus be able to catch a
stack overflow in the handler itself. Our code shouldn't cause
overflows, but it's possible that a chained handler does more work than
expected.

Pick-to: 6.3
Change-Id: I5ff8e16fcdcb4ffd9ab6fffd16eb83a294ab7958
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
This commit is contained in:
Thiago Macieira 2022-05-02 23:13:42 -07:00
parent e390ff050a
commit d5f4f91c3c
1 changed files with 67 additions and 21 deletions
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88
src/testlib/qtestcase.cpp
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@ -82,12 +82,16 @@
#include <errno.h>
#include <signal.h>
#include <time.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <sys/wait.h>
#include <unistd.h>
# if !defined(Q_OS_INTEGRITY)
# include <sys/resource.h>
# endif
# ifndef SIGSTKSZ
# define SIGSTKSZ 0 /* we have code to set the minimum */
# endif
# ifndef SA_RESETHAND
# define SA_RESETHAND 0
# endif
@ -1865,7 +1869,7 @@ public:
oldActions().fill(act);
// Remove the handler after it is invoked.
act.sa_flags = SA_RESETHAND;
act.sa_flags = SA_RESETHAND | setupAlternateStack();
# ifdef SA_SIGINFO
act.sa_flags |= SA_SIGINFO;
@ -1874,26 +1878,6 @@ public:
act.sa_handler = FatalSignalHandler::regularHandler;
# endif
// tvOS/watchOS both define SA_ONSTACK (in sys/signal.h) but mark sigaltstack() as
// unavailable (__WATCHOS_PROHIBITED __TVOS_PROHIBITED in signal.h)
# if defined(SA_ONSTACK) && !defined(Q_OS_TVOS) && !defined(Q_OS_WATCHOS)
// Let the signal handlers use an alternate stack
// This is necessary if SIGSEGV is to catch a stack overflow
# if defined(Q_CC_GNU) && defined(Q_OF_ELF)
// Put the alternate stack in the .lbss (large BSS) section so that it doesn't
// interfere with normal .bss symbols
__attribute__((section(".lbss.altstack"), aligned(4096)))
# endif
static QVarLengthArray<char, 32 * 1024> alternateStack;
alternateStack.resize(qMax(SIGSTKSZ, alternateStack.size()));
stack_t stack;
stack.ss_flags = 0;
stack.ss_size = alternateStack.size();
stack.ss_sp = alternateStack.data();
sigaltstack(&stack, nullptr);
act.sa_flags |= SA_ONSTACK;
# endif
// Block all fatal signals in our signal handler so we don't try to close
// the testlog twice.
sigemptyset(&act.sa_mask);
@ -1926,6 +1910,8 @@ public:
if (isOurs(action))
sigaction(fatalSignals[i], &act, nullptr);
}
freeAlternateStack();
}
private:
@ -1937,6 +1923,64 @@ private:
return oldActions;
}
auto alternateStackSize()
{
struct R { size_t size, pageSize; };
static constexpr size_t MinStackSize = 32 * 1024;
size_t pageSize = sysconf(_SC_PAGESIZE);
size_t size = SIGSTKSZ;
if (size < MinStackSize) {
size = MinStackSize;
} else {
// round up to a page
size = (size + pageSize - 1) & -pageSize;
}
return R{ size + pageSize, pageSize };
}
int setupAlternateStack()
{
// tvOS/watchOS both define SA_ONSTACK (in sys/signal.h) but mark sigaltstack() as
// unavailable (__WATCHOS_PROHIBITED __TVOS_PROHIBITED in signal.h)
# if defined(SA_ONSTACK) && !defined(Q_OS_TVOS) && !defined(Q_OS_WATCHOS)
// Let the signal handlers use an alternate stack
// This is necessary if SIGSEGV is to catch a stack overflow
auto r = alternateStackSize();
int flags = MAP_PRIVATE | MAP_ANONYMOUS;
# ifdef MAP_STACK
flags |= MAP_STACK;
# endif
alternateStackBase = mmap(nullptr, r.size, PROT_READ | PROT_WRITE, flags, -1, 0);
if (alternateStackBase == MAP_FAILED)
return 0;
// mark the bottom page inaccessible, to catch a handler stack overflow
(void) mprotect(alternateStackBase, r.pageSize, PROT_NONE);
stack_t stack;
stack.ss_flags = 0;
stack.ss_size = r.size - r.pageSize;
stack.ss_sp = static_cast<char *>(alternateStackBase) + r.pageSize;
sigaltstack(&stack, nullptr);
return SA_ONSTACK;
# else
return 0;
# endif
}
void freeAlternateStack()
{
# if defined(SA_ONSTACK) && !defined(Q_OS_TVOS) && !defined(Q_OS_WATCHOS)
if (alternateStackBase != MAP_FAILED) {
stack_t stack = {};
stack.ss_flags = SS_DISABLE;
sigaltstack(&stack, nullptr);
munmap(alternateStackBase, alternateStackSize().size);
}
# endif
}
static void actionHandler(int signum, siginfo_t * /* info */, void * /* ucontext */)
{
const int msecsFunctionTime = qRound(QTestLog::msecsFunctionTime());
@ -1982,6 +2026,8 @@ private:
{
actionHandler(signum, nullptr, nullptr);
}
void *alternateStackBase = MAP_FAILED;
static bool pauseOnCrash;
};
bool FatalSignalHandler::pauseOnCrash = false;