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Thu, 23 Apr 2026 14:43:41 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id UWJdBB0w6mkzegAAD6G6ig (envelope-from ) for ; Thu, 23 Apr 2026 14:43:41 +0000 From: Tom de Vries To: gdb-patches@sourceware.org Subject: [PATCH v3] [gdb] Fix segfault in sig_write Date: Thu, 23 Apr 2026 16:43:40 +0200 Message-ID: <20260423144340.3193734-1-tdevries@suse.de> X-Mailer: git-send-email 2.51.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spamd-Result: default: False [-2.80 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; MID_CONTAINS_FROM(1.00)[]; R_MISSING_CHARSET(0.50)[]; NEURAL_HAM_SHORT(-0.20)[-0.999]; MIME_GOOD(-0.10)[text/plain]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCPT_COUNT_ONE(0.00)[1]; ARC_NA(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DBL_BLOCKED_OPENRESOLVER(0.00)[sourceware.org:url,imap1.dmz-prg2.suse.org:helo,suse.de:mid]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; TO_DN_NONE(0.00)[]; PREVIOUSLY_DELIVERED(0.00)[gdb-patches@sourceware.org]; RCVD_TLS_ALL(0.00)[] X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: gdb-patches-bounces~public-inbox=simark.ca@sourceware.org I ran into a segfault in sig_write: ... if (gdb_stderr == nullptr || gdb_stderr->fd () == -1) ... [ A regression since commit 817003ed469 ("Rewrite output redirection and logging"). ] This happened as follows. First, we get gdb_stderr by calling current_gdb_stderr, which returns current_ui.current_interpreter.m_stderr.get (), which is not nullptr. Then we do "gdb_stderr->fd ()", which gets us to wrapped_file >::fd: ... int fd () const override { return m_stream->fd (); } ... The "m_stream->" part brings us to: ... /* A "smart pointer" that references a particular member of the current UI. */ template struct ui_file_ptr { ui_file *operator-> () const { return current_ui->*F; } }; ... which does "current_ui.m_ui_stderr" which indeed is nullptr. So why was current_ui.m_ui_stderr nullptr? This was due a problem fixed by commit b171f68e945 ("[gdb/tui] Make tui_setup_io more robust"). So, atm as far as we know current_ui.m_ui_stderr is never nullptr. However, sig_write has special requirements because it's called when "things go wrong", so we want it to work reliably in case things going wrong means that again current_ui.m_ui_stderr is nullptr. Fix this in wrapped_file::fd by checking for a nullptr m_stream. A v1 was submitted here [1]. Changes in v2 [2]: - changed approach of fixing this: rather than trying to fix this in sig_write by avoiding the use of gdb_stderr, fix this in wrapped_file::fd. - added unit test Changes in v3: - extended commit message to be more clear about how current_ui.m_ui_stderr got to be a nullptr Tested on x86_64-linux. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33918 [1] v1 https://sourceware.org/pipermail/gdb-patches/2026-February/225155.html [2] v2 https://sourceware.org/pipermail/gdb-patches/2026-April/226295.html --- gdb/ui-file.h | 2 +- gdb/ui.h | 4 ++++ gdb/unittests/ui-file-selftests.c | 14 ++++++++++++++ 3 files changed, 19 insertions(+), 1 deletion(-) diff --git a/gdb/ui-file.h b/gdb/ui-file.h index 6a1d3964335..b7d3aebda9a 100644 --- a/gdb/ui-file.h +++ b/gdb/ui-file.h @@ -420,7 +420,7 @@ class wrapped_file : public ui_file { m_stream->emit_style_escape (style); } int fd () const override - { return m_stream->fd (); } + { return m_stream == nullptr ? -1 : m_stream->fd (); } void puts_unfiltered (const char *str) override { m_stream->puts_unfiltered (str); } diff --git a/gdb/ui.h b/gdb/ui.h index 891660896ef..ef977470302 100644 --- a/gdb/ui.h +++ b/gdb/ui.h @@ -163,6 +163,10 @@ struct ui { return current_ui->*F; } + bool operator== (nullptr_t p) const + { + return current_ui->*F == nullptr; + } }; /* A ui_file that simply forwards. */ diff --git a/gdb/unittests/ui-file-selftests.c b/gdb/unittests/ui-file-selftests.c index 69e48735001..b9c1aca3774 100644 --- a/gdb/unittests/ui-file-selftests.c +++ b/gdb/unittests/ui-file-selftests.c @@ -48,6 +48,20 @@ run_tests () scoped_restore save_7 = make_scoped_restore (&sevenbit_strings, true); check_one ("more weird stuff: \xa5", '\\', "more weird stuff: \\245"); + + { + /* There's a bug that has the effect "*redirectable_stderr () == nullptr". + In that case, gdb_stderr is not nullptr, but the underlying pointer is + a nullptr. Check that "gdb_stderr->fd ()" doesn't dereference the + underlying nullptr. + This allows us to check for a usable gdb_stderr using + "gdb_stderr != nullptr && gdb_stderr->fd () == -1", as we do in + sig_write. */ + scoped_restore restore_stderr + = make_scoped_restore (redirectable_stderr (), nullptr); + SELF_CHECK (gdb_stderr != nullptr); + SELF_CHECK (gdb_stderr->fd () == -1); + } } } /* namespace file*/ base-commit: 8dc535c59fdbcf99e28703425e5c1c711e63a1f1 -- 2.51.0