From: owen@... Date: 2017-07-18T16:47:46+00:00 Subject: [ruby-core:82101] [Ruby trunk Bug#13753] Segfault when calling `Random::DEFAULT.rand` in forked child Issue #13753 has been updated by owst (Owen Stephens). File 0001-Re-initialise-Random-DEFAULT-after-forking-Bug-13753.patch added The attached patch fixes the issue by re-initialising the random instance such that calls to `Random::DEFAULT.rand` after forking will succeed. This now gives the same after-fork behaviour as `Random.rand`. Incidentally, there is a comment on `Random.rand` saying: ~~~ ruby * Alias of Random::DEFAULT.rand. ~~~ which is now true in the sense that they behave the same. ---------------------------------------- Bug #13753: Segfault when calling `Random::DEFAULT.rand` in forked child https://bugs.ruby-lang.org/issues/13753#change-65840 * Author: owst (Owen Stephens) * Status: Open * Priority: Normal * Assignee: * Target version: * ruby -v: ruby 2.5.0dev (2017-07-18 trunk 59362) [x86_64-linux] * Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN ---------------------------------------- Hi, The following causes a segfault on 2.3.4, 2.4.1 and current (`ruby 2.5.0dev (2017-07-18 trunk 59362) [x86_64-linux]`) ~~~ ruby puts "before fork: #{Random::DEFAULT.rand(42)}" fork do puts "child: #{Random::DEFAULT.rand(42)}" end ~~~ ~~~ /home/owen/fork_random.rb:4: [BUG] Segmentation fault at 0x0000000000000000 ruby 2.5.0dev (2017-07-18 trunk 59362) [x86_64-linux] -- Control frame information ----------------------------------------------- c:0005 p:---- s:0020 e:000019 CFUNC :rand c:0004 p:0021 s:0015 e:000012 BLOCK /home/owen/fork_random.rb:4 [FINISH] c:0003 p:---- s:0010 e:000009 CFUNC :fork c:0002 p:0033 s:0006 e:000005 EVAL /home/owen/fork_random.rb:3 [FINISH] c:0001 p:0000 s:0003 E:000790 (none) [FINISH] -- Ruby level backtrace information ---------------------------------------- /home/owen/fork_random.rb:3:in `<main>' /home/owen/fork_random.rb:3:in `fork' /home/owen/fork_random.rb:4:in `block in <main>' /home/owen/fork_random.rb:4:in `rand' -- Machine register context ------------------------------------------------ RIP: 0x00007f99e9e7eefd RBP: 0x00007fff14180190 RSP: 0x00007fff14180178 RAX: 0x0000000000000000 RBX: 0x00007f99ea17b039 RCX: 0x0000000000000004 RDX: 0x00007f99ea2af228 RDI: 0x00007f99ea2af228 RSI: 0x0000000000000029 R8: 0x0000000000000000 R9: 0x00007f99eb0b5630 R10: 0x0000000022220021 R11: 0x00007f99ea17b060 R12: 0x00007f99e9dd96a0 R13: 0x00007fff14182df0 R14: 0x00007f99eb3d3248 R15: 0x00007f99ea27af50 EFL: 0x0000000000010202 -- C level backtrace information ------------------------------------------- /home/owen/code/ruby/ruby(rb_print_backtrace+0x19) [0x7f99e9f4e081] vm_dump.c:671 /home/owen/code/ruby/ruby(rb_vm_bugreport+0xb3) [0x7f99e9f4e4fd] vm_dump.c:941 /home/owen/code/ruby/ruby(rb_bug_context+0x117) [0x7f99e9fc098e] error.c:534 /home/owen/code/ruby/ruby(sigsegv+0x60) [0x7f99e9ec8793] signal.c:930 /lib/x86_64-linux-gnu/libpthread.so.0 [0x7f99e9983330] /home/owen/code/ruby/ruby(genrand_int32+0x57) [0x7f99e9e7eefd] random.c:189 /home/owen/code/ruby/ruby(limited_rand+0xb6) [0x7f99e9e7fda1] random.c:839 /home/owen/code/ruby/ruby(random_ulong_limited+0x142) [0x7f99e9e80483] random.c:1019 /home/owen/code/ruby/ruby(rand_int+0x74) [0x7f99e9e80a09] random.c:1149 /home/owen/code/ruby/ruby(rand_random+0xe3) [0x7f99e9e811a2] random.c:1337 /home/owen/code/ruby/ruby(random_rand+0x34) [0x7f99e9e810a2] random.c:1320 /home/owen/code/ruby/ruby(call_cfunc_m1+0x2f) [0x7f99e9f2f3dd] vm_insnhelper.c:1706 /home/owen/code/ruby/ruby(vm_call_cfunc_with_frame+0x19a) [0x7f99e9f2ff10] vm_insnhelper.c:1889 ~~~ The problem is in the `rb_reset_random_seed` call that happens after forking a Thread. This function call places the `struct mt` inside `default_rand` in a partially-uninitialised state and the next call to `rand_int` (which assumes the `struct mt` _is_ initialised) tries to dereference a null pointer. N.B. the first call to `Random::DEFAULT.rand` is required to exhibit the bug else the Random instance is not placed into an inconsistent state upon forking (in particular `mt->left == 1` so the next rand call correctly re-initialises the instance) ---Files-------------------------------- 0001-Re-initialise-Random-DEFAULT-after-forking-Bug-13753.patch (1.21 KB) -- https://bugs.ruby-lang.org/ Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe> <http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>