public inbox for libstdc++-cvs@sourceware.org help / color / mirror / Atom feed
From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-9508] libstdc++: Set _M_string_length before calling _M_dispose() [PR109703] Date: Wed, 3 May 2023 13:24:39 +0000 (GMT) [thread overview] Message-ID: <20230503132439.51572385841E@sourceware.org> (raw) https://gcc.gnu.org/g:04fbfafbd9657a36e8c3e80708a18fba08136854 commit r12-9508-g04fbfafbd9657a36e8c3e80708a18fba08136854 Author: Kefu Chai <kefu.chai@scylladb.com> Date: Mon May 1 21:24:26 2023 +0100 libstdc++: Set _M_string_length before calling _M_dispose() [PR109703] This always sets _M_string_length in the constructor for ranges of input iterators, such as stream iterators. We copy from the source range to the local buffer, and then repeatedly reallocate a larger one if necessary. When disposing the old buffer, _M_is_local() is used to tell if the buffer is the local one or not (and so must be deallocated). In addition to comparing the buffer address with the local buffer, _M_is_local() has an optimization hint so that the compiler knows that for a string using the local buffer, there is an invariant that _M_string_length <= _S_local_capacity (added for PR109299 via r13-6915-gbf78b43873b0b7). But we failed to set _M_string_length in the constructor taking a pair of iterators, so the invariant might not hold, and __builtin_unreachable() is reached. This causes UBsan errors, and potentially misoptimization. To ensure the invariant holds, _M_string_length is initialized to zero before doing anything else, so that _M_is_local() doesn't see an uninitialized value. This issue only surfaces when constructing a string with a range of input iterator, and the uninitialized _M_string_length happens to be greater than _S_local_capacity, i.e., 15 for the std::string specialization. libstdc++-v3/ChangeLog: PR libstdc++/109703 * include/bits/basic_string.h (basic_string(Iter, Iter, Alloc)): Initialize _M_string_length. Signed-off-by: Kefu Chai <kefu.chai@scylladb.com> Co-authored-by: Jonathan Wakely <jwakely@redhat.com> (cherry picked from commit cbf6c7a1d16490a1e63e9a5ce00e9a5c44c4c2f2) Diff: --- libstdc++-v3/include/bits/basic_string.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h index 1dbe7c2415c..9a6fe883b6c 100644 --- a/libstdc++-v3/include/bits/basic_string.h +++ b/libstdc++-v3/include/bits/basic_string.h @@ -758,7 +758,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 _GLIBCXX20_CONSTEXPR basic_string(_InputIterator __beg, _InputIterator __end, const _Alloc& __a = _Alloc()) - : _M_dataplus(_M_local_data(), __a) + : _M_dataplus(_M_local_data(), __a), _M_string_length(0) { #if __cplusplus >= 201103L _M_construct(__beg, __end, std::__iterator_category(__beg));
reply other threads:[~2023-05-03 13:24 UTC|newest] Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20230503132439.51572385841E@sourceware.org \ --to=redi@gcc.gnu.org \ --cc=gcc-cvs@gcc.gnu.org \ --cc=libstdc++-cvs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).