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Stack overflow: stack trace and nix debugger support #8879

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Stack overflow: stack trace and nix debugger support #8879

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46 changes: 46 additions & 0 deletions src/libexpr/eval.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,7 @@

#include <sys/resource.h>
#include <nlohmann/json.hpp>
#include <pthread.h>

#if HAVE_BOEHMGC

Expand All @@ -45,6 +46,47 @@ using json = nlohmann::json;

namespace nix {

// Custom stack protection in addition to virtual memory-assisted stack overflow check (nix::sigsegvHandler)
// This has the benefit of being able to throw a proper exception, and support the debugger.

// TODO: move into evalstate
static void * stackProtectLow = nullptr;
static void * stackProtectHigh = nullptr;

// TODO: make public in evalstate. setCurrentStack?
static void initStack() {
pthread_attr_t attr;
size_t stackSize = 0;
void * stackLow = nullptr;
size_t guardSize = 0;

pthread_t self = pthread_self();
pthread_getattr_np(self, &attr);

pthread_attr_getstack(&attr, &stackLow, &stackSize);

// The guard page should be irrelevant according to POSIX, but on Linux it's within the `getstack` region!
pthread_attr_getguardsize(&attr, &guardSize);

stackProtectLow = stackLow;

// TODO: 32k seems high. Why is that?
// TODO: make this handle small stacks better?
// TODO: require a minimum stack size? The language does not define a max recursion depth, but for an evaluator to be practical, a stack that's too small is a problem that should be reported as such.
stackProtectHigh = (char*)stackLow + guardSize + std::min<size_t>(1024 * 32, stackSize / 4);
}

static inline void checkStack() {
int dummy;
void *sp = &dummy;
if (sp >= stackProtectLow && sp < stackProtectHigh) {
// No pos. A pos would be a red herring because the cause is never near the exception for stack overflows.
// TODO move into evalstate so we can do
// error("stack overflow", path).debugThrow<EvalError>();
throw EvalError("stack overflow");
}
}

static char * allocString(size_t size)
{
char * t;
Expand Down Expand Up @@ -524,6 +566,8 @@ EvalState::EvalState(

static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");

initStack();

/* Initialise the Nix expression search path. */
if (!evalSettings.pureEval) {
for (auto & i : _searchPath.elements)
Expand Down Expand Up @@ -1512,6 +1556,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
? std::make_unique<FunctionCallTrace>(positions[pos])
: nullptr;

checkStack();

forceValue(fun, pos);

Value vCur(fun);
Expand Down
2 changes: 1 addition & 1 deletion src/libutil/error.cc
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Expand Up @@ -308,7 +308,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
size_t count = 0;
for (const auto & trace : einfo.traces) {
if (trace.hint.str().empty()) continue;
if (frameOnly && !trace.frame) continue;
// if (frameOnly && !trace.frame) continue;

if (!showTrace && count > 3) {
oss << "\n" << ANSI_WARNING "(stack trace truncated; use '--show-trace' to show the full trace)" ANSI_NORMAL << "\n";
Expand Down
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