January 28, 2013

State of Chromium Open Source packages

Let me present an informal an unofficial state of Chromium Open Source packages as I see it. Note a possible bias: I'm a Chromium developer (and this post represents my views, not the projects'), and a Gentoo Linux developer (and Chromium package maintenance lead - this is a team effort, and the entire team deserves credit, especially for keeping stable and beta ebuilds up to date).

  1. Gentoo Linux - ships stable, beta and dev channels. Security updates are promptly pushed to stable. NaCl (NativeClient) is enabled, although pNaCl (Portable NaCl) is disabled. Up to 23 use_system_... gyp switches are enabled (depending on USE flags).
  2. Arch Linux - ships stable channel, promptly reacts to security updates. NaCl is enabled, following Gentoo closely - I consider that good, and I'm glad people find that code useful. :) 5 use_system_... gyp switches are enabled. A notable thing is that the PKGBUILD is one of the shortest and simplest among Chromium packages - this seems to follow from The Arch Way. There is also chromium-dev on AUR - it is more heavily based on the Gentoo package, and tracks the upstream dev channel. Uses 19 use_system_... gyp switches.
  3. FreeBSD / OpenBSD - ship stable channel, and are doing pretty well, especially when taking amount of BSD-specific patching into account. NaCl is disabled.
  4. ALT Linux - ships stable channel. NaCl seems to be disabled by default, I'm not sure what's actually shipped in compiled package. Uses 11 use_system_... gyp switches.
  5. Debian - ancient 6.x version in Squeeze, 22.x in sid at the time of this writing. This is two major milestones behind, and is missing security updates. Not recommended at this moment. :( If you are on Debian, my advice is to use Google Chrome, since official debs should work, and monitor state of the open source Chromium package. You can always return to it when it gets updated.
  6. Fedora - not in official repositories, but Tom "spot" Callaway has an unofficial repo. Note: currently the version in that repo is 23.x, one major version behind on stable. Tom wrote an article in 2009 called Chromium: Why it isn't in Fedora yet as a proper package, so there is definitely an interest to get it packaged for Fedora, which I appreciate. Many of the issues he wrote about are now fixed, and I hope to work on getting the remaining ones fixed. Please stay tuned!
This is not intended to be an exhaustive list. I'm aware of openSUSE packages, there seems to be something happening for Ubuntu, and I've heard of Slackware, Pardus, PCLinuxOS and CentOS packaging. I do not follow these closely enough though to provide a meaningful "review".

Some conclusions: different distros package Chromium differently. Pay attention to the packaging lag: with about 6 weeks upstream release cycle and each major update being a security one, this matters. Support for NativeClient is another point. There are extension and Web Store apps that use it, and when more and more sites start to use it, this will become increasingly important. Then it is interesting why on some distros some bundled libraries are used even though upstream provides an option to use a system library that is known to work on other distros.

Finally, I like how different maintainers look at each other's packages, and how patches and bugs are frequently being sent upstream.

January 4, 2013

Signal handler safety, re-entering malloc

This is a story from real-world development. From signal(7):

   Async-signal-safe functions
       A  signal  handler  function must be very careful,
       since processing elsewhere may be interrupted at some
       arbitrary point in the execution of the program.
       POSIX has the concept of "safe function".  If a signal
       interrupts the execution of an  unsafe  function,
       and handler calls an unsafe function, then the behavior
       of the program is undefined.

After that a list of safe functions follows, and one notable things is that malloc and free are async-signal-unsafe!

I hit this issue while enabling tcmalloc's debugallocation for Chromium Debug builds. We have a StackDumpSignalHandler for tests, which prints a stack trace on various crashing signals for easier debugging. It's very useful, and worked fine for a pretty long while (which means that "but it works!" is not a valid argument for doing unsafe things).

Now when I enabled debugallocation, I noticed hangs triggered by the stack trace display. In one example, this stack trace:

@0  0x00000000019c6c85 in tcmalloc::Abort () at third_party/tcmalloc/chromium/src/base/abort.cc:15
@1  0x00000000019b39c1 in LogPrintf (severity=-4,
    pat=0x32aeb18 "memory allocation/deallocation mismatch at %p: allocated with %s being deallocated with %s", ap=0x7fff52c379e8)
    at third_party/tcmalloc/chromium/src/base/logging.h:210
@2  0x00000000019b3a8b in RAW_LOG (lvl=-4,
    pat=0x32aeb18 "memory allocation/deallocation mismatch at %p: allocated with %s being deallocated with %s")
    at third_party/tcmalloc/chromium/src/base/logging.h:230
@3  0x00000000019c3fb1 in MallocBlock::CheckLocked (this=0x7fd18f143400, type=-21308287)
    at ./third_party/tcmalloc/chromium/src/debugallocation.cc:461
@4  0x00000000019c3c42 in MallocBlock::CheckAndClear (this=0x7fd18f143400, type=-21308287)
    at ./third_party/tcmalloc/chromium/src/debugallocation.cc:401
@5  0x00000000019c436a in MallocBlock::Deallocate (this=0x7fd18f143400, type=-21308287)
    at ./third_party/tcmalloc/chromium/src/debugallocation.cc:557
@6  0x00000000019c1929 in DebugDeallocate (ptr=0x7fd18f143420, type=-21308287)
    at ./third_party/tcmalloc/chromium/src/debugallocation.cc:998
@7  0x00000000028d1482 in tc_delete (p=0x7fd18f143420) at ./third_party/tcmalloc/chromium/src/debugallocation.cc:1232
@8  0x000000000097dc04 in cc::ResourceProvider::deleteResourceInternal (this=0x7fd191827da0, it=...) at cc/resource_provider.cc:242
@9  0x000000000097daaf in cc::ResourceProvider::deleteResource (this=0x7fd191827da0, id=1) at cc/resource_provider.cc:230
@10 0x00000000006f9824 in (anonymous namespace)::ResourceProviderTest_Basic_Test::TestBody (this=0x7fd18dc5abf0)
    at cc/resource_provider_unittest.cc:328
@11 0x00000000008ec801 in testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void> (object=0x7fd18dc5abf0,
    method=&virtual testing::Test::TestBody(), location=0x29463ab "the test body") at testing/gtest/src/gtest.cc:2071
@12 0x00000000008e9665 in testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void> (object=0x7fd18dc5abf0,
    method=&virtual testing::Test::TestBody(), location=0x29463ab "the test body") at testing/gtest/src/gtest.cc:2123
@13 0x00000000008dee0d in testing::Test::Run (this=0x7fd18dc5abf0) at testing/gtest/src/gtest.cc:2143
@14 0x00000000008df3ea in testing::TestInfo::Run (this=0x7fd191823020) at testing/gtest/src/gtest.cc:2319
@15 0x00000000008df8dc in testing::TestCase::Run (this=0x7fd19181f0d0) at testing/gtest/src/gtest.cc:2426
@16 0x00000000008e3eea in testing::internal::UnitTestImpl::RunAllTests (this=0x7fd19829dd60) at testing/gtest/src/gtest.cc:4249

generates SIGSEGV (tcmalloc::Abort). This is just debugallocation having stricter checks about usage of dynamically allocated memory. Now the StackDumpSignalHandler kicks in, and internally calls malloc. But we're already inside malloc code as you can see on the above stack trace (see frame @7, bold font), and re-entering it tries to take locks that are already held, resulting in a hang.

The fix required several changes:
  • no dynamic memory, and that includes std::string and std::vector, which use it internally
  • no buffered stdio or iostreams, they are not async-signal-safe (that includes fflush)
  • custom code for number-to-string conversion that doesn't need dynamically allocated memory (snprintf is not on the list of safe functions as of POSIX.1-2008; it seems to work on a glibc-2.15-based system, but as said before this is not a good assumption to make); in this code I've named it itoa_r, and it supports both base-10 and base-16 conversions, and also negative numbers for base-10
  • warming up backtrace(3): now this is really tricky, and backtrace(3) itself is not whitelisted for being safe; in fact, on the very first call it does some memory allocations; for now I've just added a call to backtrace() from a context that is safe and happens before the signal handler may be executed; implementing backtrace(3) in a known-safe way would be another fun thing to do
Note that for the above, I've also added a unit test that triggers the deadlock scenario. This will hopefully catch cases where calling backtrace(3) leads to trouble.

For more info, feel free to read the articles below: