[libc] Add nullptr check option to printf %s

[michaelrj]

Summary

Some printf implementations perform a null check on pointers passed to
%s. While that's not in the standard, this patch adds it as an option
for compatibility. It also puts a similar check in %n behind the same
flag.

[lntue]

libc/test/src/stdio/sprintf_test.cpp

101

Also add the expectation for this output when LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS is set.

2791

Also add the expectation for this output when LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS is set.

[michaelrj]

libc/test/src/stdio/sprintf_test.cpp

101

If the flag isn't set, the expectation is for it to segfault so it would have to be a death test. I'm not sure that's a useful test though.

[mcgrathr]

libc/src/stdio/printf_core/string_converter.h

29

IIRC the traditional string is "(null)", not "null".
That's what glibc does.
Also note for %p it uses "(nil)" instead, so go figure.
Also glibc only writes "(null)" at all if the precision allows the full string, never truncates it to "(nu" for %.3s. If it doesn't fit, it just writes nothing instead (as for an argument of "").

libc/src/stdio/printf_core/printf_config.h

 // LIBC_COPT_FLOAT_TO_STR_USE_DYADIC_FLOAT
 // LIBC_COPT_FLOAT_TO_STR_USE_DYADIC_FLOAT_LD
 // LIBC_COPT_FLOAT_TO_STR_USE_INT_CALC
 // LIBC_COPT_FLOAT_TO_STR_NO_TABLE
 // LIBC_COPT_PRINTF_HEX_LONG_DOUBLE
 
 // TODO(michaelrj): Move the other printf configuration options into this file.
 
+// LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+
 #endif // LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PRINTF_CONFIG_H

libc/src/stdio/printf_core/string_converter.h

 
 #include <stddef.h>
 
 namespace __llvm_libc {
 namespace printf_core {
 
 LIBC_INLINE int convert_string(Writer *writer, const FormatSection &to_conv) {
   size_t string_len = 0;
+  const char *str_ptr = reinterpret_cast<const char *>(to_conv.conv_val_ptr);
 
-  for (char *cur_str = reinterpret_cast<char *>(to_conv.conv_val_ptr);
-       cur_str[string_len]; ++string_len) {
+#ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+  if (str_ptr == nullptr) {
+    str_ptr = "null";
+  }
+#endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+
+  for (const char *cur_str = (str_ptr); cur_str[string_len]; ++string_len) {
     ;
   }
 
   if (to_conv.precision >= 0 &&
       static_cast<size_t>(to_conv.precision) < string_len)
     string_len = to_conv.precision;
 
   size_t padding_spaces = to_conv.min_width > static_cast<int>(string_len)
                               ? to_conv.min_width - string_len
                               : 0;
 
   // If the padding is on the left side, write the spaces first.
   if (padding_spaces > 0 &&
       (to_conv.flags & FormatFlags::LEFT_JUSTIFIED) == 0) {
     RET_IF_RESULT_NEGATIVE(writer->write(' ', padding_spaces));
   }
 
-  RET_IF_RESULT_NEGATIVE(writer->write(
-      {reinterpret_cast<const char *>(to_conv.conv_val_ptr), string_len}));
+  RET_IF_RESULT_NEGATIVE(writer->write({(str_ptr), string_len}));
 
   // If the padding is on the right side, write the spaces last.
   if (padding_spaces > 0 &&
       (to_conv.flags & FormatFlags::LEFT_JUSTIFIED) != 0) {
     RET_IF_RESULT_NEGATIVE(writer->write(' ', padding_spaces));
   }
   return WRITE_OK;
 }
 
 } // namespace printf_core
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_STDIO_PRINTF_CORE_STRING_CONVERTER_H

libc/src/stdio/printf_core/write_int_converter.h

 #include <stddef.h>
 
 namespace __llvm_libc {
 namespace printf_core {
 
 LIBC_INLINE int convert_write_int(Writer *writer,
                                   const FormatSection &to_conv) {
 
-  // This is an additional check added by LLVM-libc. The reason it returns -3 is
-  // because printf uses negative return values for errors, and -1 and -2 are
-  // already in use by the file_writer class for file errors.
-  // TODO: Remove this. It's better to crash than to provide an incorrect
-  // result.
+#ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+  // This is an additional check added by LLVM-libc.
   if (to_conv.conv_val_ptr == nullptr)
     return NULLPTR_WRITE_ERROR;
+#endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
 
   int written = writer->get_chars_written();
 
   switch (to_conv.length_modifier) {
   case LengthModifier::none:
     *reinterpret_cast<int *>(to_conv.conv_val_ptr) = written;
     break;
   case LengthModifier::l:

libc/test/src/stdio/sprintf_test.cpp

written = __llvm_libc::sprintf(buff, "%-5.4s%-4.4s", "words can describe",
                                  "soups most delicious");
   EXPECT_EQ(written, 9);
   ASSERT_STREQ(buff, "word soup");
 
   written = __llvm_libc::sprintf(buff, "%*s %.*s %*.*s", 10, "beginning", 2,
                                  "isn't", 12, 10, "important. Ever.");
   EXPECT_EQ(written, 26);
   ASSERT_STREQ(buff, " beginning is   important.");
+
+#ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+  written = __llvm_libc::sprintf(buff, "%s", nullptr);
+  EXPECT_EQ(written, 4);
+  ASSERT_STREQ(buff, "null");
+#endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
 }
 
 TEST(LlvmLibcSPrintfTest, IntConv) {
   char buff[64];
   int written;
 
   // Basic Tests.
 
TEST(LlvmLibcSPrintfTest, WriteIntConv) {
   ASSERT_STREQ(buff, "ABCDEF");
 
   test_val = -1;
   written = __llvm_libc::sprintf(buff, "%s%n", "87654321", &test_val);
   EXPECT_EQ(written, 8);
   EXPECT_EQ(test_val, 8);
   ASSERT_STREQ(buff, "87654321");
 
+#ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
   written = __llvm_libc::sprintf(buff, "abc123%n", nullptr);
   EXPECT_LT(written, 0);
+#endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
 }
 #endif // LIBC_COPT_PRINTF_DISABLE_WRITE_INT
 
 #ifndef LIBC_COPT_PRINTF_DISABLE_INDEX_MODE
 TEST(LlvmLibcSPrintfTest, IndexModeParsing) {
   char buff[64];
   int written;