CompilerFlags

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== Ignoring return code of write() ==

If the error is ignoring the return code of write() or a similar function then it indicates that the
code is not implementing retry support for writes. write(2) says

{{{
       If a write() is interrupted by a signal handler before any bytes are
       written, then the call fails with the error EINTR; if it is interrupted
       after at least one byte has been written, the call succeeds, and
       returns the number of bytes written.
}}}

So something like the following should be used to retry when there is a short write
(thanks to SteveLangasek)

{{{
  do {
      written = write(fd, buf, count);
      if (written == count)
          break;
      if (written > 0) {
          buf += written;
          count -= written;
      }
  } while (written >= 0 || errno == EINTR);
  if (written < 0) {
    /* Handle error */
  }
}}}

This page documents the Ubuntu-specific default compiler flags in the toolchain. Based on the work from GccSsp, Security/HardeningWrapper, and DistCompilerFlags. Please attempt to fix a source package's problems before disabling a given compiler feature, and document the package and bug numbers in the Problems section below.

Default Flags

-fstack-protector

First enabled in Ubuntu 6.10. Enabled run-time stack overflow verification. See GccSsp for further details. Most problems are related to packages that do not use stdlib directly (kernel modules, certain libraries, etc).

Failure examples:

  • '__stack_chk_fail' symbol not found
    • Indicates a program was compiled to expect to have the stdlib available, but did not find it at runtime.
    *** stack smashing detected ***
    Aborted
    • A function did not correctly maintain its stack variables. Usually indicates a stack buffer overflow.

Disabled with -fno-stack-protector or -nostdlib in CPPFLAGS.

-D_FORTIFY_SOURCE=2

First enabled in Ubuntu 8.10. Provides compile-time best-practices errors for certain libc functions, and provides run-time checks of buffer lengths and memory regions. Only activated when compiled with -O2 or higher. Most problems are related to common unsafe uses of certain libc functions. (For implementation details, see [http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html])

Failure examples:

  • error: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result
    • The return value from system(), fwrite(), and similar functions should be evaluated and handled appropriately. In cases where one absolutely must throw away the return value, it can be discarded with an empty test: if (system("...")) { } , though this is not recommended.

    error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT in second argument needs 3 arguments
    • When using open() with O_CREAT, best-practice is to define a valid mode argument. For the least modes, try using (S_IRUSR|S_IWUSR) first. If that doesn't work as expected in the program, then start adding back perms. For example, user and group: (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP); user, group, and other: (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH).

    warning: call to ‘__read_chk_warn’ declared with attribute warning: read called with bigger length than size of the destination buffer
    • The call to read() was done into a buffer with the wrong size. Double-check the size argument and the buffer size.

    warning: passing argument 1 of 'memcpy' discards qualifiers from pointer target type
    warning: passing argument 1 of 'strcpy' discards qualifiers from pointer target type
    *** %n in writable segment detected ***
    Aborted
    • On x86, use of "%n" in a format string is limited to read-only memory (not stack or heap allocated strings).

    *** buffer overflow detected ***
    Aborted
    • A call to sprintf should be changed to use snprintf, or a too-small buffer was read into (see read() warnings above).

Reduced checking via -D_FORTIFY_SOURCE=1 in CPPFLAGS. Disabled with -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0 in CPPFLAGS.

-Wformat -Wformat-security

First enabled in Ubuntu 8.10. Enables compile-time warnings about misuse of format strings, some of which can have security implications. These options should only cause build failures if the package is compiling with -Werror.

Failure examples:

  • warning: format ‘%s’ expects type ‘char *’, but argument 3 has type ‘int’
    • For packages that aren't already building with -Wall, format character to argument types will be checked. Verify the correct variables for a given format string.

    warning: format not a string literal and no format arguments
    • This is caused by code that forgot to use "%s" for a *printf function. For example:

      • fprintf(stderr,buf);
      should be:
      • fprintf(stderr,"%s",buf);

Disabled with -Wno-format-security or -Wformat=0 in CPPFLAGS.

-Wl,-z,relro

First enabled in Ubuntu 8.10. Provides a read-only relocation table area in the final ELF. This option paves the way for using -z now which forces all relocations to be resolved at run-time (which would cause some additional initial load delay), providing an even higher level of protection to the relocation table -- it could then be entirely read-only which can be used to further harden long-running programs like daemons.

No known failure examples.

Disabled with -Wl,-z,norelro in LDFLAGS.

Triage

When encountering "ignoring return value of ..." build problems due to the FORTIFY flag, it is best to follow this general approach (and to refer to the above individual sections on a case-by-case basis):

  1. If there is are obvious error handlers in near-by code, emulate them. An example can be seen with conntrack. This is, obviously, the preferred method of dealing with it.

  2. If there isn't an obvious way to handle the error, then stubbing out an empty handler is the next way to go -- fundamentally, this doesn't improve (or weaken) the quality of the code -- ignoring the return code is what's already happening, so this doesn't change anything. However, what it _does_ do, is allows the FORTIFY option to still be enabled, which means the program will gain the run-time FORTIFY protections.
  3. If there isn't a way to work around the error, and the problem is isolated to a small area of the source code, I've disabled FORTIFY for _only_ those source files, which can be a pain, depending on the package's build methods.

  4. If nothing works, and other people have looked at it, and no one has any ideas about how to work around a problem with FORTIFY, then it's time to disable it for the entire build using the -U_FORTIFY_SOURCE CFLAG.

Now, in all of these situations, upstream needs to be notified. Especially for the #2, where they need to make a larger design decision about how to deal with the unhandled error condition.

Also, in situations where you've had to disable FORTIFY (#3, #4), please document the issue in the "Problems" section below (preferably also with a bug).

Ignoring return code of write()

If the error is ignoring the return code of write() or a similar function then it indicates that the code is not implementing retry support for writes. write(2) says

       If  a  write()  is interrupted by a signal handler before any bytes are
       written, then the call fails with the error EINTR; if it is interrupted
       after  at  least  one  byte  has  been  written, the call succeeds, and
       returns the number of bytes written.

So something like the following should be used to retry when there is a short write (thanks to SteveLangasek)

  do {
      written = write(fd, buf, count);
      if (written == count)
          break;
      if (written > 0) {
          buf += written;
          count -= written;
      }
  } while (written >= 0 || errno == EINTR);
  if (written < 0) {
    /* Handle error */
  }

Problems

If the upstream source cannot be reasonably fixed and a package must have compiler flags disabled or some other work-around, please open a launchpad bug, tag it with "hardening-ftbfs", and link to it here along with an explanation of what the problem is:

bacula

binutils

  • Add -Wno-format-security to CFLAGS, CXXFLAGS, due to picky default hardening options.

binutils-avr

gcc-4.3

  • Testsuite reports false failures due to warnings, etc.

gdb

  • Testsuite reports false failures due to warnings, etc.

icmake

libxfont1

sysklogd

synergy

thunderbird

ToolChain/CompilerFlags (last edited 2021-09-02 05:51:07 by paelzer)