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Fix UB in format_arg_store implementation. #3833

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merged 1 commit into from
Jan 31, 2024
Merged

Fix UB in format_arg_store implementation. #3833

merged 1 commit into from
Jan 31, 2024

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ivafanas
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format_arg_store struct contains pointers to its own members, so autogenerated copy constructor and copy assignment is not correct. Need to reassign pointers to |this| internals.

Compiler won't autogenerate incorrect move constructor and move assignment, because user-provided copy constructor and copy assignment member functions exist.

vitaut
vitaut requested changes Jan 31, 2024
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Thanks for the PR. format_arg_store is not supposed to be copied so we should delete copy and assignment operators instead.

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Ok.
PR is updated.

@ivafanas
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Many reports from CI:

https://github.com/fmtlib/fmt/actions/runs/7728340058/job/21068957883?pr=3833

/home/runner/work/fmt/fmt/include/fmt/xchar.h:91:51: error: use of deleted function ‘fmt::v10::detail::format_arg_store<Context, NUM_ARGS, NUM_NAMED_ARGS, DESC>::format_arg_store(fmt::v10::detail::format_arg_store<Context, NUM_ARGS, NUM_NAMED_ARGS, DESC>&&) [with Context = fmt::v10::generic_context<fmt::v10::basic_appender<wchar_t>, wchar_t>; long unsigned int NUM_ARGS = 3; long unsigned int NUM_NAMED_ARGS = 3; long long unsigned int DESC = 4611686018427388364]’
   91 |   return make_format_args<wformat_context>(args...);

https://github.com/fmtlib/fmt/actions/runs/7728340053/job/21068956966?pr=3833

/Users/runner/work/fmt/fmt/include/fmt/xchar.h:91:10: error: call to deleted constructor of 'decltype(make_format_args<fmt::wformat_context>(args, args, args))' (aka 'format_arg_store<fmt::generic_context<fmt::basic_appender<wchar_t>, wchar_t>, sizeof...(T), 3UL, 4611686018427388364ULL>')
  return make_format_args<wformat_context>(args...);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

https://github.com/fmtlib/fmt/actions/runs/7728340056/job/21068959707?pr=3833

D:\a\fmt\fmt\include\fmt/xchar.h(91): error C2280: 'fmt::v10::detail::format_arg_store<fmt::v10::wformat_context,3,3,4611686018427388364>::format_arg_store(fmt::v10::detail::format_arg_store<fmt::v10::wformat_context,3,3,4611686018427388364> &&)': attempting to reference a deleted function [D:\a\fmt\build\test\xchar-test.vcxproj]

etc.

So, copy constructor must exist. Copy assignment operator might be deleted, I suggest.

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vitaut commented Jan 31, 2024

So, copy constructor must exist. Copy assignment operator might be deleted, I suggest.

Yeah, let's go with that.

phprus
phprus reviewed Jan 31, 2024
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include/fmt/base.h Outdated
@@ -1636,7 +1636,9 @@ template <typename Context, size_t NUM_ARGS, size_t NUM_NAMED_ARGS,
struct format_arg_store {
// args_[0].named_args points to named_args to avoid bloating format_args.
// +1 to workaround a bug in gcc 7.5 that causes duplicated-branches warning.
arg_t<Context, NUM_ARGS> args[1 + (NUM_ARGS != 0 ? NUM_ARGS : +1)];
static constexpr int ARGS_ARR_SIZE = 1 + (NUM_ARGS != 0 ? NUM_ARGS : +1);
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Maybe size_t (and i in copy constructor)?
Like size_t NUM_ARGS, size_t NUM_NAMED_ARGS template arguments.

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Seems like loops optimization prevention for unsigned iterator due to allowed overflow is not a case here, because loop bounds are known at compile time.

Yep, size_t is a good option here.

@ivafanas ivafanas force-pushed the fix_ub_in_format_arg_store branch 2 times, most recently from 955f509 to 311ffbd Compare January 31, 2024 16:41
vitaut
vitaut reviewed Jan 31, 2024
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include/fmt/base.h Outdated
Comment on lines 1655 to 1660
format_arg_store(const format_arg_store& rhs) {
args[0] = {named_args, NUM_NAMED_ARGS};
for (size_t i = 1; i < ARGS_ARR_SIZE; ++i) args[i] = rhs.args[i];
for (size_t i = 0; i < NUM_NAMED_ARGS; ++i)
named_args[i] = rhs.named_args[i];
}
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I think we only need move ctor.

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Updated

@vitaut vitaut merged commit 1b54ba4 into fmtlib:master Jan 31, 2024
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vitaut commented Jan 31, 2024

Thank you!

happymonkey1 pushed a commit to happymonkey1/fmt that referenced this pull request Apr 7, 2024
@ivafanas @happymonkey1
Fix UB in format_arg_store implementation. (fmtlib#3833)
4409d8f
Tradias
Tradias reviewed Jul 1, 2024
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include/fmt/base.h
@@ -1649,6 +1651,17 @@ struct format_arg_store {
0,
(init_named_arg(named_args, arg_index, named_arg_index, values), 0)...};
}

format_arg_store(format_arg_store&& rhs) {
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This move constructor should be noexcept

@BrewTestBot BrewTestBot mentioned this pull request Jul 1, 2024
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@vitaut vitaut vitaut approved these changes

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4 participants
@ivafanas @vitaut @phprus @Tradias