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[104.155.91.135]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-499740a6f4esm48747265e9.2.2026.08.11.02.09.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 11 Aug 2026 02:09:30 -0700 (PDT) Date: Tue, 11 Aug 2026 10:09:27 +0100 From: Vincent Donnefort To: Steven Rostedt Cc: mhiramat@kernel.org, linux-trace-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com, kernel-team@android.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 06/18] tracing/remotes: Add dmesg tracefs file Message-ID: References: <20260804140333.797210-1-vdonnefort@google.com> <20260804140333.797210-7-vdonnefort@google.com> <20260810184216.3484cfd6@robin> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260810184216.3484cfd6@robin> On Mon, Aug 10, 2026 at 06:42:16PM -0400, Steven Rostedt wrote: > Adding Sashiko review: https://sashiko.dev/#/patchset/20260804140333.797210-2-vdonnefort%40google.com > > > --- Patch [6]: [PATCH v4 06/18] tracing/remotes: Add dmesg tracefs file --- > > commit b6a4c19257d91807c612baa19691b2d602bb0670 > > Author: Vincent Donnefort > > > > tracing/remotes: Add dmesg tracefs file > > > > When enabled, the dmesg tracefs file enables the redirection of all > > events to dmesg. This is similar to tp_printk. > > > > > diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c > > > index 1fbe2f3f20e99..58a38f3d49024 100644 > > > --- a/kernel/trace/trace_remote.c > > > +++ b/kernel/trace/trace_remote.c > > > > > [ ... ] > > > > > @@ -882,6 +908,87 @@ static const struct file_operations trace_fops = { > > > .release = trace_release, > > > }; > > > > > > +static bool trace_remote_do_dmesg(struct trace_remote *remote) > > > +{ > > > + struct trace_remote_iterator *iter = smp_load_acquire(&remote->dmesg); > > > + unsigned int max_events = 1000; > > > + > > > + if (!iter) > > > + return false; > > > + > > > + trace_remote_iter_read_start(iter); > > > + > > > + while (trace_remote_iter_read_event(iter)) { > > > + trace_seq_init(&iter->seq); > > > + > > > + trace_remote_iter_print_event(iter); > > > + trace_seq_putc(&iter->seq, 0); > > > + if (!pr_info("%s", iter->seq.buffer)) > > > > If the remote trace event exceeds the PAGE_SIZE sequence buffer, does > > trace_seq_putc() in trace_remote_do_dmesg() fail to append the null byte? > > > > Since iter->seq.buffer might not be null-terminated in this case, could this > > lead to an out-of-bounds read when pr_info() prints the string? > > Yeah, we may want to have it be: > > if (!pr_info("%s", seq_buf_str(&iter->seq)) > > Which will make sure the buffer is nul terminated. ack, and same for trace_remote_panic_handler() > > > > > > This isn't a bug introduced by this patch, but the same pattern appears > > to exist in the pre-existing trace_remote_panic_handler() with pr_emerg(). > > > > If the kernel is built with CONFIG_PRINTK disabled, pr_info() evaluates to 0. > > > > Will this cause the loop in trace_remote_do_dmesg() to break without calling > > trace_remote_iter_move(), leaving the event unconsumed and permanently > > stalling the workqueue? > > > > > + break; > > > + > > > + trace_remote_iter_move(iter); > > > + > > > + if (!(--max_events)) > > > + break; > > > + } > > > + > > > + trace_remote_iter_read_finished(iter); > > > + > > > + return !max_events; > > > +} > > > > While reviewing this, I also noticed a pre-existing issue in trace_pipe_read() > > when handling oversized events: > > > > kernel/trace/trace_remote.c:trace_pipe_read() { > > ... > > if (trace_remote_iter_print_event(iter)) { > > iter->seq.seq.len = prev_len; > > break; > > } > > ... > > } > > > > This is a pre-existing issue, but if an oversized event causes > > trace_remote_iter_print_event() to return -EOVERFLOW, will this break > > the loop without calling trace_remote_iter_move()? > > > > Since the event is left unconsumed in the buffer, could this lead to an > > uninterruptible infinite loop (hard lockup) when reading the trace pipe? > > I'll let you answer the above. > > -- Steve I don't believe this can realistically happen, trace_seq buffer is 8K and remote events are really small. -- Vincent