breakpad doesn't create a minidump when CONFIG_PAX_RANDMMAP is enabled

[Nikita Kozlov]

Hello,

I was looking at why we had no minidumps for some of our crashes on some
servers and I found that it was because of the CONFIG_PAX_RANDMMAP Linux
kernel features from the PaX patchset.
I don't know if it was reported yet or if there is a known workaround for it.

I have tested it with several kernels, from 3.14.X to 4.2.X on gentoo and
debian.
My test program look like this :
```
newCExceptionHandlerSimple();
// from https://github.com/Gandi/breakpad-c-wrapper, C wrapper around
// google_breakpad::ExceptionHandler
fd = open(argv[1], O_RDONLY);
addr = mmap(NULL, 1, PROT_READ, MAP_PRIVATE, fd, 0);
ftruncate(fd, 0);
fd = getchar();
printf("%x\n", addr[0x4]); //SIGBUS
```
the output is :

```
$ LD_PRELOAD=./libgbc_c_wrapper.so ./sigbus emptyfile
ExceptionHandler::DoDump google_breakpad::WriteMinidump without FD
Dump path: /tmp/0599f86e-902f-a14d-27ee0962-2a123136.dmp
zsh: segmentation fault (core dumped) LD_PRELOAD=.//libgbc_c_wrapper.so ./sigbus emptyfile
```

The core dump doesn't appear to have anything useful strangely, the
stack is from my program and not breakpad.
Without CONFIG_PAX_RANDMMAP everything is working as expected.

I didn't took a look yet on where it was failing, I'm unfamiliar with
breakpad code and it look difficult to debug since I cannot attach a
debugger to it (or I'm missing something?).

Any idea on how to start to debug without a lot of printf or on where to
start looking would be really appreciated :)

Regards,

-- Nikita

[Mike Frysinger]

attaching a debugger should work fine even when using the in-process handlers

can you try disabling the handlers entirely and look at the coredump you get back, with & w/out randmmap enabled ?  then see if the coredumps have all the mappings you need.

you might want to try posting to the grsecurity forums too as they have much more experience with the kernel settings and possible non-obvious changes in behavior.

-mike

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