Improve gcore and support dumping ELF headers

Back in 2016, when life was simpler, a Fedora GDB user reported a bug (or a feature request, depending on how you interpret it) saying that GDB’s gcore command did not respect the COREFILTER_ELF_HEADERS flag, which instructs it to dump memory pages containing ELF headers. As you may or may not remember, I have already written about the broader topic of revamping GDB’s internal corefile dump algorithm; it’s an interesting read and I recommend it if you don’t know how Linux (or GDB) decides which mappings to dump to a corefile.

Anyway, even though the bug was interesting and had to do with a work I’d done before, I couldn’t really work on it at the time, so I decided to put it in the TODO list. Of course, the “TODO list” is actually a crack where most things fall through and are usually never seen again, so I was blissfully ignoring this request because I had other major priorities to deal with. That is, until a seemingly unrelated problem forced me to face this once and for all!

What? A regression? Since when?

As the Fedora GDB maintainer, I’m routinely preparing new releases for Fedora Rawhide distribution, and sometimes for the stable versions of the distro as well. And I try to be very careful when dealing with new releases, because a regression introduced now can come and bite us (i.e., the Red Hat GDB team) back many years in the future, when it’s sometimes too late or too difficult to fix things. So, a mandatory part of every release preparation is to actually run a regression test against the previous release, and make sure that everything is working correctly.

One of these days, some weeks ago, I had finished running the regression check for the release I was preparing when I noticed something strange: a specific, Fedora-only corefile test was FAILing. That’s a no-no, so I started investigating and found that the underlying reason was that, when the corefile was being generated, the build-id note from the executable was not being copied over. Fedora GDB has a local patch whose job is to, given a corefile with a build-id note, locate the corresponding binary that generated it. Without the build-id note, no binary was being located.

Coincidentally or not, at the same I started noticing some users reporting very similar build-id issues on the freenode’s #gdb channel, and I thought that this bug had a potential to become a big headache for us if nothing was done to fix it right now.

I asked for some help from the team, and we managed to discover that the problem was also happening with upstream gcore, and that it was probably something that binutils was doing, and not GDB. Hmm…

Ah, so it’s ld’s fault. Or is it?

So there I went, trying to confirm that it was binutils’s fault, and not GDB’s. Of course, if I could confirm this, then I could also tell the binutils guys to fix it, which meant less work for us :-).

With a lot of help from Keith Seitz, I was able to bisect the problem and found that it started with the following commit:

commit f6aec96dce1ddbd8961a3aa8a2925db2021719bb
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Tue Feb 27 11:34:20 2018 -0800

	ld: Add --enable-separate-code

This is a commit that touches the linker, which is part of binutils. So that means this is not GDB’s problem, right?!? Hmm. No, unfortunately not.

What the commit above does is to simply enable the use of --enable-separate-code (or -z separate-code) by default when linking an ELF program on x86_64 (more on that later). On a first glance, this change should not impact the corefile generation, and indeed, if you tell the Linux kernel to generate a corefile (for example, by doing sleep 60 & and then hitting C-\), you will notice that the build-id note is included into it! So GDB was still a suspect here. The investigation needed to continue.

What’s with -z separate-code?

The -z separate-code option makes the code segment in the ELF file to put in a completely separated segment than data segment. This was done to increase the security of generated binaries. Before it, everything (code and data) was put together in the same memory region. What this means in practice is that, before, you would see something like this when you examined /proc/PID/smaps:

00400000-00401000 r-xp 00000000 fc:01 798593                             /file
Size:                  4 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                   4 kB
Pss:                   4 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         4 kB
Referenced:            4 kB
Anonymous:             4 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
THPeligible:    0
VmFlags: rd ex mr mw me dw sd

And now, you will see two memory regions instead, like this:

00400000-00401000 r--p 00000000 fc:01 799548                             /file
Size:                  4 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                   4 kB
Pss:                   4 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         4 kB
Private_Dirty:         0 kB
Referenced:            4 kB
Anonymous:             0 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
THPeligible:    0
VmFlags: rd mr mw me dw sd
00401000-00402000 r-xp 00001000 fc:01 799548                             /file
Size:                  4 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                   4 kB
Pss:                   4 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         4 kB
Referenced:            4 kB
Anonymous:             4 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
THPeligible:    0
VmFlags: rd ex mr mw me dw sd

A few minor things have changed, but the most important of them is the fact that, before, the whole memory region had anonymous data in it, which means that it was considered an anonymous private mapping (anonymous because of the non-zero Anonymous amount of data; private because of the p in the r-xp permission bits). After -z separate-code was made default, the first memory mapping does not have Anonymous contents anymore, which means that it is now considered to be a file-backed private mapping instead.

GDB, corefile, and coredump_filter

It is important to mention that, unlike the Linux kernel, GDB doesn’t have all of the necessary information readily available to decide the exact type of a memory mapping, so when I revamped this code back in 2015 I had to create some heuristics to try and determine this information. If you’re curious, take a look at the linux-tdep.c file on GDB’s source tree, specifically at the functions dump_mapping_p and linux_find_memory_regions_full.

When GDB is deciding which memory regions should be dumped into the corefile, it respects the value found at the /proc/PID/coredump_filter file. The default value for this file is 0x33, which, according to core(5), means:

Dump memory pages that are either anonymous private, anonymous
shared, ELF headers or HugeTLB.

GDB had the support implemented to dump almost all of these pages, except for the ELF headers variety. And, as you can probably infer, this means that, before the -z separate-code change, the very first memory mapping of the executable was being dumped, because it was marked as anonymous private. However, after the change, the first mapping (which contains only data, no code) wasn’t being dumped anymore, because it was now considered by GDB to be a file-backed private mapping!

Finally, that is the reason for the difference between corefiles generated by GDB and Linux, and also the reason why the build-id note was not being included in the corefile anymore! You see, the first memory mapping contains not only the program’s data, but also its ELF headers, which in turn contain the build-id information.

gcore, meet ELF headers

The solution was “simple”: I needed to improve the current heuristics and teach GDB how to determine if a mapping contains an ELF header or not. For that, I chose to follow the Linux kernel’s algorithm, which basically checks the first 4 bytes of the mapping and compares them against \177ELF, which is ELF’s magic number. If the comparison succeeds, then we just assume we’re dealing with a mapping that contains an ELF header and dump it.

In all fairness, Linux just dumps the first page (4K) of the mapping, in order to save space. It would be possible to make GDB do the same, but I chose the faster way and just dumped the whole mapping, which, in most scenarios, shouldn’t be a big problem.

It’s also interesting to mention that GDB will just perform this check if:

• The heuristic has decided not to dump the mapping so far, and;
• The mapping is private, and;
• The mapping’s offset is zero, and;
• There is a request to dump mappings with ELF headers (i.e., coredump_filter).

Linux also makes these checks, by the way.

The patch, finally

I submitted the patch to the mailing list, and it was approved fairly quickly (with a few minor nits).

The reason I’m writing this blog post is because I’m very happy and proud with the whole process. It wasn’t an easy task to investigate the underlying reason for the build-id failures, and it was interesting to come up with a solution that extended the work I did a few years ago. I was also able to close a few bug reports upstream, as well as the one reported against Fedora GDB.

The patch has been pushed, and is also present at the latest version of Fedora GDB for Rawhide. It wasn’t possible to write a self-contained testcase for this problem, so I had to resort to using an external tool (eu-unstrip) in order to guarantee that the build-id note is correctly present in the corefile. But that’s a small detail, of course.

Anyway, I hope this was an interesting (albeit large) read!