> On 11 Mar 2019, at 18:16, Tom Callaway <tcall...@redhat.com> wrote:
>
> Hi folks,
>
> I spent some time this weekend trying to get Chromium 72 building on Fedora,
> but I kept running into a C++ issue that I was not able to resolve. This
> happened with gcc-9.0.1-0.8.fc30.x86_64 and gcc-8.3.1-2.fc29.x86_64.
>
> Here's a sample of the error (it happens in a few places), from Fedora 30:
>
> In file included from
> ../../base/trace_event/trace_event_system_stats_monitor.h:15,
> from ../../base/trace_event/trace_event.h:26,
> from ../../base/threading/scoped_blocking_call.cc:11:
> ../../base/trace_event/trace_log.h: In constructor
> 'base::ScopedBlockingCall::ScopedBlockingCall(base::BlockingType)':
> ../../base/threading/scoped_blocking_call.cc:88:5: in 'constexpr' expansion
> of 'base::trace_event::TraceLog::GetBuiltinCategoryEnabled(((const
> char*)"base"))'
> ../../base/trace_event/trace_log.h:215:25: error: '((&
> base::trace_event::CategoryRegistry::categories_[7]) != 0)' is not a constant
> expression
It looks like the compiler is not smart enough to detect that this is the
address of a static object, defined at line 92744 of your preprocessed file as:
static TraceCategory categories_[kMaxCategories];
whereas it is smart enough to go through GetBuiltnCategoryByName and figure out
index 7 from the name…
I tried to compile your preprocessed fragment with both clang or gcc.
Interestingly:
1) Without your command-line options, I don’t see the same error.
2) With your command-line options, I see the error with gcc, but not with clang.
So I suspect a compiler bug.
> 215 | if (builtin_category)
> | ^
>
> Now, chromium isn't the easiest code base to work with, but what seems to be
> happening is that this code calls one of the TRACE_EVENT macros, like this
> from base/threading/scoped_blocking_call.cc:
>
> TRACE_EVENT_BEGIN1("base", "ScopedBlockingCall", "blocking_type",
> static_cast<int>(blocking_type));
>
> Those macros are defined in base/trace_event/common/trace_event_common.h:
>
> #define TRACE_EVENT_BEGIN1(category_group, name, arg1_name, arg1_val) \
> INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, category_group, name, \
> TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
>
> INTERNAL_TRACE_EVENT_ADD() is defined in base/trace_event/trace_event.h:
>
> / Implementation detail: internal macro to create static category and add
> // event if the category is enabled.
> #define INTERNAL_TRACE_EVENT_ADD(phase, category_group, name, flags, ...) \
> do { \
> INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category_group); \
> if (INTERNAL_TRACE_EVENT_CATEGORY_GROUP_ENABLED()) { \
> trace_event_internal::AddTraceEvent( \
> phase, INTERNAL_TRACE_EVENT_UID(category_group_enabled), name, \
> trace_event_internal::kGlobalScope, trace_event_internal::kNoId, \
> flags, trace_event_internal::kNoId, ##__VA_ARGS__); \
> } \
> } while (0)
>
> INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO is also defined in
> base/trace_event/trace_event.h:
>
> #define INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category_group)
> \
> static_assert(
> \
> base::trace_event::BuiltinCategories::IsAllowedCategory(category_group),
> \
> "Unknown tracing category is used. Please register your "
> \
> "category in base/trace_event/builtin_categories.h");
> \
> constexpr const unsigned char* INTERNAL_TRACE_EVENT_UID(
> \
> k_category_group_enabled) =
> \
> base::trace_event::TraceLog::GetBuiltinCategoryEnabled(category_group);
> \
> const unsigned char* INTERNAL_TRACE_EVENT_UID(category_group_enabled);
> \
> INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO_MAYBE_AT_COMPILE_TIME(
> \
> category_group, INTERNAL_TRACE_EVENT_UID(k_category_group_enabled),
> \
> INTERNAL_TRACE_EVENT_UID(category_group_enabled));
>
> Finally, inside here, it calls
> base::trace_event::TraceLog::GetBuiltinCategoryEnabled(), which is defined in
> base/trace_event/trace_log.h:
>
> // Called by TRACE_EVENT* macros, don't call this directly.
> // The name parameter is a category group for example:
> // TRACE_EVENT0("renderer,webkit", "WebViewImpl::HandleInputEvent")
> static const unsigned char* GetCategoryGroupEnabled(const char* name);
> static const char* GetCategoryGroupName(
> const unsigned char* category_group_enabled);
> static constexpr const unsigned char* GetBuiltinCategoryEnabled(
> const char* name) {
> TraceCategory* builtin_category =
> CategoryRegistry::GetBuiltinCategoryByName(name);
> if (builtin_category)
> return builtin_category->state_ptr();
> return nullptr;
> }
>
> Okay, so what seems like is happening here, is that the calls like this:
>
> TRACE_EVENT_BEGIN1("base", "ScopedBlockingCall", "blocking_type",
> static_cast<int>(blocking_type));
>
> Are passing "base" (that first var) all the way into GetCategoryGroupEnabled,
> which is finding it via GetBuiltinCategoryByName() in
> base/trace_event/category_registry.h, checking against the list in
> INTERNAL_TRACE_LIST_BUILTIN_CATEGORIES(X) from
> base/trace_event/builtin_categories.h, finding it, then returning this as
> builtin_category:
>
> (& base::trace_event::CategoryRegistry::categories_[7])
>
> When if(builtin_category) is run (trying to check to see if we got a
> buillt-in category or a nullptr, I think), thats when GCC says:
>
> error: '((& base::trace_event::CategoryRegistry::categories_[7]) != 0)' is
> not a constant expression
>
> *****
>
> None of the other distros that build Chromium seem to have hit this issue,
> nor does it appear in any bugs I could find with upstream (but they use a
> fork of clang to build, and we use gcc). I have hit the limit of my C++
> knowledge here, so I'm not sure how to fix this. If you can help me, it would
> be greatly appreciated. My work in progress is checked into the master
> chromium branch in git.
>
> There are quite a few security issues that this new version fixes, including
> some that are being actively exploited, so timely help here is appreciated.
>
> Thanks,
>
> ~tom
>
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