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platform_bionic/docs/status.md
Elliott Hughes a7119af33e Update libc status documentation. 
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Change-Id: Ied9659f0388a4affa65f0948b3e4a58d0dca5605
2018-07-17 13:26:14 -07:00

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Android bionic status

Bionic function availability

POSIX

You can see the current status with respect to POSIX in the form of tests: https://android.googlesource.com/platform/bionic/+/master/tests/headers/posix/

Some POSIX functionality is not supported by the Linux kernel, and is guarded with tests for __linux__. Other functionality is not supported by bionic or glibc, and guarded with tests for __BIONIC__ and __GLIBC__. In other cases historical accidents mean 32-bit bionic diverged but 64-bit bionic matches POSIX; these are guarded with __LP64__.

Most bionic-only diversions should be accompanied by an explanatory comment.

Missing functions are either obsolete or explicitly disallowed by SELinux:

  • a64l/l64a
  • confstr
  • crypt/encrypt/setkey
  • gethostid
  • shm_open/shm_unlink
  • sockatmark

Missing functionality:

  • <aio.h>
  • <wordexp.h>
  • Thread cancellation
  • Robust mutexes

Run ./libc/tools/check-symbols-glibc.py in bionic/ for the current list of POSIX functions implemented by glibc but not by bionic.

libc

Current libc symbols: https://android.googlesource.com/platform/bionic/+/master/libc/libc.map.txt

New libc functions in Q:

  • timespec_get (C11 <time.h> addition)
  • res_randomid (in <resolv.h>)

New libc functions in P:

  • __freading/__fwriting (completing <stdio_ext.h>)
  • endhostent/endnetent/endprotoent/getnetent/getprotoent/sethostent/setnetent/setprotoent (completing <netdb.h>)
  • fexecve
  • fflush_unlocked/fgetc_unlocked/fgets_unlocked/fputc_unlocked/fputs_unlocked/fread_unlocked/fwrite_unlocked
  • getentropy/getrandom (adding <sys/random.h>)
  • getlogin_r
  • glob/globfree (adding <glob.h>)
  • hcreate/hcreate_r/hdestroy/hdestroy_r/hsearch/hsearch_r (completing <search.h>)
  • iconv/iconv_close/iconv_open (adding <iconv.h>)
  • pthread_attr_getinheritsched/pthread_attr_setinheritsched/pthread_setschedprio
  • pthread_mutexattr_getprotocol/pthread_mutexattr_setprotocol (mutex priority inheritance)
  • <signal.h> support for sigaction64_t and sigset64_t allowing LP32 access to real-time signals
  • <spawn.h>
  • swab
  • syncfs

New libc behavior in P:

  • %C and %S support in the printf family (previously only the wprintf family supported these)
  • %mc/%ms/%m[ support in the scanf family
  • %s support in strptime (strftime already supported it)

New libc functions in O:

  • sendto FORTIFY support
  • __system_property_read_callback/__system_property_wait
  • legacy bsd_signal
  • catclose/catgets/catopen (adding <nl_types.h>)
  • ctermid
  • all 6 <grp.h>/<pwd.h> (get|set|end)(gr|pw)ent functions
  • futimes/futimesat/lutimes
  • getdomainname/setdomainname
  • getsubopt
  • hasmntopt
  • mallopt
  • mblen
  • 4 <sys/msg.h> msg* functions
  • <langinfo.h> nl_langinfo/nl_langinfo_l
  • pthread_getname_np
  • 2 new Linux system calls quotactl and sync_file_range
  • 4 <sys/sem.h> sem* functions
  • 4 <sys/shm.h> shm* functions
  • 5 legacy <signal.h> functions: sighold/sigignore/sigpause/sigrelse/sigset
  • strtod_l/strtof_l/strtol_l/strtoul_l
  • <wctype.h> towctrans/towctrans_l/wctrans/wctrans_l

New libc functions in N:

  • more FORTIFY support functions (fread/fwrite/getcwd/pwrite/write)
  • all remaining _FILE_OFFSET_BITS=64 functions, completing _FILE_OFFSET_BITS=64 support in bionic (8)
  • all 7 pthread_barrier* functions
  • all 5 pthread_spin* functions
  • lockf/preadv/pwritev/scandirat and off64_t variants
  • adjtimex/clock_adjtime
  • getifaddrs/freeifaddrs/if_freenameindex/if_nameindex
  • getgrgid_r/getgrnam_r
  • GNU extensions fileno_unlocked/strchrnul
  • 32-bit prlimit

libc function count over time: G 803, H 825, I 826, J 846, J-MR1 873, J-MR2 881, K 896, L 1116, M 1181, N 1226, O 1278

ndk-r17$ for i in `ls -1v platforms/android-*/arch-arm/usr/lib/libc.so` ; do \
  echo $i; nm $i | grep -vw [AbdNnt] | grep -vw B | wc -l ; done

libm

Current libm symbols: https://android.googlesource.com/platform/bionic/+/master/libm/libm.map.txt

0 remaining missing POSIX libm functions.

19 new libm functions in O: complex trig/exp/log functions.

libm function count over time: G 158, J-MR2 164, L 220, M 265, O 284

Target API level behavioral differences

Most bionic bug fixes and improvements have been made without checks for the app's targetSdkVersion. As of O there were exactly two exceptions, but there are likely to be more in future because of Project Treble.

Invalid pthread_t handling (targetSdkVersion >= O)

As part of a long-term goal to remove the global thread list, and in an attempt to flush out racy code, we changed how an invalid pthread_t is handled. For pthread_detach, pthread_getcpuclockid, pthread_getschedparam/pthread_setschedparam, pthread_join, and pthread_kill, instead of returning ESRCH when passed an invalid pthread_t, if you're targeting O or above, they'll abort with the message "attempt to use invalid pthread_t".

Note that this doesn't change behavior as much as you might think: the old lookup only held the global thread list lock for the duration of the lookup, so there was still a race between that and the dereference in the caller, given that callers actually need the tid to pass to some syscall or other, and sometimes update fields in the pthread_internal_t struct too.

We can't check a thread's tid against 0 to see whether a pthread_t is still valid because a dead thread gets its thread struct unmapped along with its stack, so the dereference isn't safe.

To fix your code, taking the affected functions one by one:

  • pthread_getcpuclockid and pthread_getschedparam/pthread_setschedparam should be fine. Unsafe calls to those seem highly unlikely.

  • Unsafe pthread_detach callers probably want to switch to pthread_attr_setdetachstate instead, or use pthread_detach(pthread_self()); from the new thread's start routine rather than calling detach in the parent.

  • pthread_join calls should be safe anyway, because a joinable thread won't actually exit and unmap until it's joined. If you're joining an unjoinable thread, the fix is to stop marking it detached. If you're joining an already-joined thread, you need to rethink your design!

  • Unsafe pthread_kill calls aren't portably fixable. (And are obviously inherently non-portable as-is.) The best alternative on Android is to use pthread_gettid_np at some point that you know the thread to be alive, and then call kill/tgkill with signal 0 (which checks whether a process exists rather than actually sending a signal). That's still not completely safe because if you're too late the tid may have been reused, but your code is inherently unsafe without a redesign anyway.

Interruptable sem_wait (targetSdkVersion >= N)

POSIX says that sem_wait can be interrupted by delivery of a signal. This wasn't historically true in Android, and when we fixed this bug we found that existing code relied on the old behavior. To preserve compatibility, sem_wait can only return EINTR on Android if the app targets N or later.

FORTIFY

The _FORTIFY_SOURCE macro can be used to enable extra automatic bounds checking for common libc functions. If a buffer overrun is detected, the program is safely aborted as in this (example)[https://source.android.com/devices/tech/debug/native-crash#fortify].

Note that in recent releases Android's FORTIFY has been extended to cover other issues. It can now detect, for example, passing O_CREAT to open(2) without specifying a mode. It also performs some checking regardless of whether the caller was built with FORTIFY enabled. In P, for example, calling a pthread_mutex_ function on a destroyed mutex, calling a <dirent.h> function on a null pointer, using %n with the printf(3) family, or using the scanf(3) m modifier incorrectly will all result in FORTIFY failures even for code not built with FORTIFY.

More background information is available in our (FORTIFY in Android)[https://android-developers.googleblog.com/2017/04/fortify-in-android.html] blog post.

The Android platform is built with -D_FORTIFY_SOURCE=2, but NDK users need to manually enable FORTIFY by setting that themselves in whatever build system they're using. The exact subset of FORTIFY available to NDK users will depend on their target ABI level, because when a FORTIFY check can't be guaranteed at compile-time, a call to a run-time _chk function is added.