linux-kernel - [PATCH 1/3] tracing: Remove dependency of saved_cmdlines_buffer on PID_MAX

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Date: Mon,  8 Apr 2024 16:58:17 +0200
From: Michal Koutný <mkoutny@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Christian Brauner <brauner@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Kees Cook <keescook@...omium.org>,
	Michal Koutný <mkoutny@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tycho Andersen <tandersen@...flix.com>,
	Jens Axboe <axboe@...nel.dk>,
	Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH 1/3] tracing: Remove dependency of saved_cmdlines_buffer on PID_MAX_DEFAULT

Calculations into map_pid_to_cmdline use PID_MAX_DEFAULT but they
actually depend on the size of map_pid_to_cmdline. The size of the map
may be arbitrary. First, refer to the map size where necessary, second,
pick a good value for the size of the map.
Since the buffer is allocated at boot (i.e. user cannot affect its size
later), accounting for full PID_MAX_LIMIT would inflate map's size
unnecessarily (4*4M) for all users. Stick to the original value of
4*32k, the commit 785e3c0a3a87 ("tracing: Map all PIDs to command
lines") explains why it still works for higher pids.

The point of this exercise is to remove dependency on PID_MAX_DEFAULT.

Signed-off-by: Michal Koutný <mkoutny@...e.com>
---
 kernel/trace/trace_sched_switch.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
index 8a407adb0e1c..aca2dafdd97a 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -161,6 +161,7 @@ static size_t tgid_map_max;
 
 #define SAVED_CMDLINES_DEFAULT 128
 #define NO_CMDLINE_MAP UINT_MAX
+#define PID_MAP_SIZE (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)
 /*
  * Preemption must be disabled before acquiring trace_cmdline_lock.
  * The various trace_arrays' max_lock must be acquired in a context
@@ -168,7 +169,7 @@ static size_t tgid_map_max;
  */
 static arch_spinlock_t trace_cmdline_lock = __ARCH_SPIN_LOCK_UNLOCKED;
 struct saved_cmdlines_buffer {
-	unsigned map_pid_to_cmdline[PID_MAX_DEFAULT+1];
+	unsigned map_pid_to_cmdline[PID_MAP_SIZE];
 	unsigned *map_cmdline_to_pid;
 	unsigned cmdline_num;
 	int cmdline_idx;
@@ -248,7 +249,7 @@ int trace_save_cmdline(struct task_struct *tsk)
 	if (!tsk->pid)
 		return 1;
 
-	tpid = tsk->pid & (PID_MAX_DEFAULT - 1);
+	tpid = tsk->pid % PID_MAP_SIZE;
 
 	/*
 	 * It's not the end of the world if we don't get
@@ -294,7 +295,7 @@ static void __trace_find_cmdline(int pid, char comm[])
 		return;
 	}
 
-	tpid = pid & (PID_MAX_DEFAULT - 1);
+	tpid = pid % PID_MAP_SIZE;
 	map = savedcmd->map_pid_to_cmdline[tpid];
 	if (map != NO_CMDLINE_MAP) {
 		tpid = savedcmd->map_cmdline_to_pid[map];
@@ -645,8 +646,8 @@ tracing_saved_cmdlines_size_write(struct file *filp, const char __user *ubuf,
 	if (ret)
 		return ret;
 
-	/* must have at least 1 entry or less than PID_MAX_DEFAULT */
-	if (!val || val > PID_MAX_DEFAULT)
+	/* must have at least 1 entry or fit into map_pid_to_cmdline */
+	if (!val || val >= PID_MAP_SIZE)
 		return -EINVAL;
 
 	ret = tracing_resize_saved_cmdlines((unsigned int)val);
-- 
2.44.0