MOZ_CRASH(seccomp+tsync failed, but kernel supports tsync) when recording Firefox

[theres-waldo]

STR:

1. Install rr
2. Run a local build of mozilla-central with |mach run --debugger rr|
3. Load any webpage

Expected results:
The page loads.

Actual results:
The content process crashes, with the following output:

Sandbox: thread-synchronized seccomp failed: Function not implemented
Hit MOZ_CRASH(seccomp+tsync failed, but kernel supports tsync) at /home/botond/dev/mozilla/central/security/sandbox/linux/Sandbox.cpp:452

Originally filed as https://bugzilla.mozilla.org/show_bug.cgi?id=1298869, where :jld diagnosed it as an rr bug, hence I'm filing it here now.

[rocallahan]

What kernel version and Gecko changeset are you using?

[theres-waldo]

Kernel version: 3.16.0-4-amd64
Gecko changeset: https://hg.mozilla.org/mozilla-central/rev/8ef9629d8f90 + unrelated local patches

[jld]

FYI, SECCOMP_FILTER_FLAG_TSYNC first appeared in 3.17 (torvalds/linux@c2e1f2e) but some distributions backported it (e.g., in Ubuntu Trusty it goes back to the tag Ubuntu-3.13.0-40.67). The seccomp syscall is part of the same patch set. This problem seems to need a kernel that doesn't support the seccomp syscall; there are more details on the Bugzilla bug.

[theres-waldo]

rr with this patch does not build for me:

/home/botond/dev/mozilla/rr/obj/32/seccomp_tsync.c: In function ‘install_filter’:
/home/botond/dev/mozilla/rr/obj/32/seccomp_tsync.c:17:54: error: ‘SECCOMP_FILTER_FLAG_TSYNC’ undeclared (first use in this function)
   ret = syscall(RR_seccomp, SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC,
                                                      ^
/home/botond/dev/mozilla/rr/obj/32/seccomp_tsync.c:17:54: note: each undeclared identifier is reported only once for each function it appears in
/home/botond/dev/mozilla/rr/rr/src/test/seccomp_tsync.c: In function ‘install_filter’:
/home/botond/dev/mozilla/rr/rr/src/test/seccomp_tsync.c:17:54: error: ‘SECCOMP_FILTER_FLAG_TSYNC’ undeclared (first use in this function)
   ret = syscall(RR_seccomp, SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC,
                                                      ^
/home/botond/dev/mozilla/rr/rr/src/test/seccomp_tsync.c:17:54: note: each undeclared identifier is reported only once for each function it appears in

[rocallahan]

Hopefully fixed by 60aa4ca

[theres-waldo]

Almost:

/home/botond/dev/mozilla/rr/rr/src/SeccompFilterRewriter.cc: In function ‘void rr::install_patched_seccomp_filter_arch(rr::RecordTask*, std::unordered_map<unsigned int, short unsigned int>&, std::vector<unsigned int>&)’:
/home/botond/dev/mozilla/rr/rr/src/SeccompFilterRewriter.cc:111:29: error: ‘SECCOMP_FILTER_FLAG_TSYNC’ was not declared in this scope
         (t->regs().arg2() & SECCOMP_FILTER_FLAG_TSYNC)) {
                             ^

[rocallahan]

Argh! Try again.

[theres-waldo]

Compiles now, and fixes the crash. Thanks!