Avoid fprintf in Settings_write

Settings_write() is called on a crash inside of a signal handler.  Using
fprintf(3) is not safe to call ins signal handlers due to file buffering
and memory allocations.
Format messages in signal handlers via snprintf(3) and output them via
write(2) on crash.

1 files changed, 53 insertions, 29 deletions

diff --git a/Settings.c b/Settings.c
index e2c94e2b..7bbf6a45 100644
--- a/Settings.c
+++ b/Settings.c
@@ -563,65 +563,88 @@ static bool Settings_read(Settings* this, const char* fileName, unsigned int ini
    return didReadAny;
 }
 
-static void writeFields(FILE* fd, const ProcessField* fields, Hashtable* columns, bool byName, char separator) {
+typedef ATTR_FORMAT(printf, 2, 3) int (*OutputFunc)(FILE*, const char*,...);
+
+static void writeFields(OutputFunc of, FILE* fd,
+                        const ProcessField* fields, Hashtable* columns,
+                        bool byName, char separator) {
    const char* sep = "";
    for (unsigned int i = 0; fields[i]; i++) {
       if (fields[i] < LAST_PROCESSFIELD && byName) {
          const char* pName = toFieldName(columns, fields[i], NULL);
-         fprintf(fd, "%s%s", sep, pName);
+         of(fd, "%s%s", sep, pName);
       } else if (fields[i] >= LAST_PROCESSFIELD && byName) {
          bool enabled;
          const char* pName = toFieldName(columns, fields[i], &enabled);
          if (enabled)
-            fprintf(fd, "%sDynamic(%s)", sep, pName);
+            of(fd, "%sDynamic(%s)", sep, pName);
       } else {
          // This "-1" is for compatibility with the older enum format.
-         fprintf(fd, "%s%d", sep, (int) fields[i] - 1);
+         of(fd, "%s%d", sep, (int) fields[i] - 1);
       }
       sep = " ";
    }
-   fputc(separator, fd);
+   of(fd, "%c", separator);
 }
 
-static void writeList(FILE* fd, char** list, int len, char separator) {
+static void writeList(OutputFunc of, FILE* fd,
+                      char** list, int len, char separator) {
    const char* sep = "";
    for (int i = 0; i < len; i++) {
-      fprintf(fd, "%s%s", sep, list[i]);
+      of(fd, "%s%s", sep, list[i]);
       sep = " ";
    }
-   fputc(separator, fd);
+   of(fd, "%c", separator);
 }
 
-static void writeMeters(const Settings* this, FILE* fd, char separator, unsigned int column) {
+static void writeMeters(const Settings* this, OutputFunc of,
+                        FILE* fd, char separator, unsigned int column) {
    if (this->hColumns[column].len) {
-      writeList(fd, this->hColumns[column].names, this->hColumns[column].len, separator);
+      writeList(of, fd, this->hColumns[column].names, this->hColumns[column].len, separator);
    } else {
-      fputc('!', fd);
-      fputc(separator, fd);
+      of(fd, "!%c", separator);
    }
 }
 
-static void writeMeterModes(const Settings* this, FILE* fd, char separator, unsigned int column) {
+static void writeMeterModes(const Settings* this, OutputFunc of,
+                            FILE* fd, char separator, unsigned int column) {
    if (this->hColumns[column].len) {
       const char* sep = "";
       for (size_t i = 0; i < this->hColumns[column].len; i++) {
-         fprintf(fd, "%s%d", sep, this->hColumns[column].modes[i]);
+         of(fd, "%s%d", sep, this->hColumns[column].modes[i]);
          sep = " ";
       }
    } else {
-      fputc('!', fd);
+      of(fd, "!");
    }
 
-   fputc(separator, fd);
+   of(fd, "%c", separator);
+}
+
+ATTR_FORMAT(printf, 2, 3)
+static int signal_safe_fprintf(FILE* stream, const char* fmt, ...) {
+   char buf[2048];
+
+   va_list vl;
+   va_start(vl, fmt);
+   int n = vsnprintf(buf, sizeof(buf), fmt, vl);
+   va_end(vl);
+
+   if (n <= 0)
+      return n;
+
+   return full_write_str(fileno(stream), buf);
 }
 
 int Settings_write(const Settings* this, bool onCrash) {
    FILE* fd;
    char separator;
    char* tmpFilename = NULL;
+   OutputFunc of;
    if (onCrash) {
       fd = stderr;
       separator = ';';
+      of = signal_safe_fprintf;
    } else {
       /* create tempfile with mode 0600 */
       mode_t cur_umask = umask(S_IXUSR | S_IRWXG | S_IRWXO);
@@ -634,20 +657,21 @@ int Settings_write(const Settings* this, bool onCrash) {
       if (fd == NULL)
          return -errno;
       separator = '\n';
+      of = fprintf;
    }
 
    #define printSettingInteger(setting_, value_) \
-      fprintf(fd, setting_ "=%d%c", (int) (value_), separator)
+      of(fd, setting_ "=%d%c", (int) (value_), separator)
    #define printSettingString(setting_, value_) \
-      fprintf(fd, setting_ "=%s%c", value_, separator)
+      of(fd, setting_ "=%s%c", value_, separator)
 
    if (!onCrash) {
-      fprintf(fd, "# Beware! This file is rewritten by htop when settings are changed in the interface.\n");
-      fprintf(fd, "# The parser is also very primitive, and not human-friendly.\n");
+      of(fd, "# Beware! This file is rewritten by htop when settings are changed in the interface.\n");
+      of(fd, "# The parser is also very primitive, and not human-friendly.\n");
    }
    printSettingString("htop_version", VERSION);
    printSettingInteger("config_reader_min_version", CONFIG_READER_MIN_VERSION);
-   fprintf(fd, "fields="); writeFields(fd, this->screens[0]->fields, this->dynamicColumns, false, separator);
+   of(fd, "fields="); writeFields(of, fd, this->screens[0]->fields, this->dynamicColumns, false, separator);
    printSettingInteger("hide_kernel_threads", this->hideKernelThreads);
    printSettingInteger("hide_userland_threads", this->hideUserlandThreads);
    printSettingInteger("hide_running_in_container", this->hideRunningInContainer);
@@ -688,10 +712,10 @@ int Settings_write(const Settings* this, bool onCrash) {
 
    printSettingString("header_layout", HeaderLayout_getName(this->hLayout));
    for (unsigned int i = 0; i < HeaderLayout_getColumns(this->hLayout); i++) {
-      fprintf(fd, "column_meters_%u=", i);
-      writeMeters(this, fd, separator, i);
-      fprintf(fd, "column_meter_modes_%u=", i);
-      writeMeterModes(this, fd, separator, i);
+      of(fd, "column_meters_%u=", i);
+      writeMeters(this, of, fd, separator, i);
+      of(fd, "column_meter_modes_%u=", i);
+      writeMeterModes(this, of, fd, separator, i);
    }
 
    // Legacy compatibility with older versions of htop
@@ -709,14 +733,14 @@ int Settings_write(const Settings* this, bool onCrash) {
       const char* sortKey = toFieldName(this->dynamicColumns, ss->sortKey, NULL);
       const char* treeSortKey = toFieldName(this->dynamicColumns, ss->treeSortKey, NULL);
 
-      fprintf(fd, "screen:%s=", ss->heading);
-      writeFields(fd, ss->fields, this->dynamicColumns, true, separator);
+      of(fd, "screen:%s=", ss->heading);
+      writeFields(of, fd, ss->fields, this->dynamicColumns, true, separator);
       if (ss->dynamic) {
          printSettingString(".dynamic", ss->dynamic);
          if (ss->sortKey && ss->sortKey != PID)
-            fprintf(fd, "%s=Dynamic(%s)%c", ".sort_key", sortKey, separator);
+            of(fd, "%s=Dynamic(%s)%c", ".sort_key", sortKey, separator);
          if (ss->treeSortKey && ss->treeSortKey != PID)
-            fprintf(fd, "%s=Dynamic(%s)%c", ".tree_sort_key", treeSortKey, separator);
+            of(fd, "%s=Dynamic(%s)%c", ".tree_sort_key", treeSortKey, separator);
       } else {
          printSettingString(".sort_key", sortKey);
          printSettingString(".tree_sort_key", treeSortKey);