From 07496eafb0166aafd9c33a6a95e16bcbc64c34d4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rin=20Cat=20=28=E9=88=B4=E7=8C=AB=29?= <dev@rincat.ch>
Date: Mon, 8 Nov 2021 09:42:12 -0500
Subject: Linux: use proper way to detect kernel threads

Use PF_KTHREAD flag in /proc/[pid]/stat to detect kernel threads.
This fixed an issue when a process's cmdline is empty, htop think
it is a kernel thread.
---
 linux/LinuxProcess.h     |  3 +++
 linux/LinuxProcessList.c | 35 ++++++++++++++++++++---------------
 2 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/linux/LinuxProcess.h b/linux/LinuxProcess.h
index 577b903d..b4e1a8bf 100644
--- a/linux/LinuxProcess.h
+++ b/linux/LinuxProcess.h
@@ -48,6 +48,9 @@ typedef struct LinuxProcess_ {
    long m_drs;
    long m_lrs;
 
+   /* Process flags */
+   unsigned long int flags;
+
    /* Data read (in bytes) */
    unsigned long long io_rchar;
 
diff --git a/linux/LinuxProcessList.c b/linux/LinuxProcessList.c
index 377dd7e6..a2d3d2be 100644
--- a/linux/LinuxProcessList.c
+++ b/linux/LinuxProcessList.c
@@ -62,6 +62,10 @@ in the source distribution for its full text.
 #define O_PATH         010000000 // declare for ancient glibc versions
 #endif
 
+/* Not exposed yet. Defined at include/linux/sched.h */
+#ifndef PF_KTHREAD
+#define PF_KTHREAD 0x00200000
+#endif
 
 static long long btime = -1;
 
@@ -374,8 +378,9 @@ static bool LinuxProcessList_readStatFile(Process* process, openat_arg_t procFd,
    process->tpgid = strtol(location, &location, 10);
    location += 1;
 
-   /* Skip (9) flags  -  %u */
-   location = strchr(location, ' ') + 1;
+   /* (9) flags  -  %u */
+   lp->flags = strtoul(location, &location, 10);
+   location += 1;
 
    /* (10) minflt  -  %lu */
    process->minflt = strtoull(location, &location, 10);
@@ -1110,17 +1115,9 @@ delayacct_failure:
 static bool LinuxProcessList_readCmdlineFile(Process* process, openat_arg_t procFd) {
    char command[4096 + 1]; // max cmdline length on Linux
    ssize_t amtRead = xReadfileat(procFd, "cmdline", command, sizeof(command));
-   if (amtRead < 0)
+   if (amtRead <= 0)
       return false;
 
-   if (amtRead == 0) {
-      if (process->state != ZOMBIE) {
-         process->isKernelThread = true;
-      }
-      Process_updateCmdline(process, NULL, 0, 0);
-      return true;
-   }
-
    int tokenEnd = 0;
    int tokenStart = 0;
    int lastChar = 0;
@@ -1488,6 +1485,10 @@ static bool LinuxProcessList_recurseProcTree(LinuxProcessList* this, openat_arg_
       if (! LinuxProcessList_readStatFile(proc, procFd, statCommand, sizeof(statCommand)))
          goto errorReadingProcess;
 
+      if (lp->flags & PF_KTHREAD) {
+         proc->isKernelThread = true;
+      }
+
       if (tty_nr != proc->tty_nr && this->ttyDrivers) {
          free(proc->tty_name);
          proc->tty_name = LinuxProcessList_updateTtyDevice(this->ttyDrivers, proc->tty_nr);
@@ -1519,8 +1520,10 @@ static bool LinuxProcessList_recurseProcTree(LinuxProcessList* this, openat_arg_
          }
          #endif
 
-         if (! LinuxProcessList_readCmdlineFile(proc, procFd)) {
-            goto errorReadingProcess;
+         if (proc->isKernelThread) {
+            Process_updateCmdline(proc, NULL, 0, 0);
+         } else if (!LinuxProcessList_readCmdlineFile(proc, procFd)) {
+            Process_updateCmdline(proc, statCommand, 0, strlen(statCommand));
          }
 
          Process_fillStarttimeBuffer(proc);
@@ -1528,8 +1531,10 @@ static bool LinuxProcessList_recurseProcTree(LinuxProcessList* this, openat_arg_
          ProcessList_add(pl, proc);
       } else {
          if (settings->updateProcessNames && proc->state != ZOMBIE) {
-            if (! LinuxProcessList_readCmdlineFile(proc, procFd)) {
-               goto errorReadingProcess;
+            if (proc->isKernelThread) {
+               Process_updateCmdline(proc, NULL, 0, 0);
+            } else if (!LinuxProcessList_readCmdlineFile(proc, procFd)) {
+               Process_updateCmdline(proc, statCommand, 0, strlen(statCommand));
             }
          }
       }
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