Memory mappings, core dumps, GDB and Linux

After spending the last weeks struggling with this, I decided to write a blog post. First, what is “this” that you are talking about? The answer is: Linux kernel’s concept of memory mapping. I found it utterly confused, beyond my expectations, and so I believe that a blog post is the write way to (a) preserve and (b) share this knowledge. So, let’s do it!

First things first

First, I cannot begin this post without a few acknowledgements and “thank you’s”. The first goes to Oleg Nesterov (sorry, I could not find his website), a Linux kernel guru who really helped me a lot through the whole task. Another “thank you” goes to Jan Kratochvil, who also provided valuable feedback by commenting my GDB patch. Now, back to the point.

The task

The task was requested here: GDB needed to respect the /proc/<PID>/coredump_filter file when generating a coredump (i.e., when you use the gcore command).

Currently, GDB has his own coredump mechanism implemented which, despite its limitations and bugs, has been around for quite some time. However, and maybe you don’t know that, but the Linux kernel has its own algorithm for generating the corefile of a process. And unfortunately, GDB and Linux were not really following the same standards here…

So, in the end, the task was about synchronizing GDB and Linux. To do that, I first had to decipher the contents of the /proc/<PID>/smaps file.

The /proc/<PID>/smaps file

This special file, generated by the Linux kernel when you read it, contains detailed information about each memory mapping of a certain process. Some of the fields on this file are documented in the proc(5) manpage, but others are missing there (asking for a patch!). Here is an explanation of everything I needed:

• The first line of each memory mapping has the following format:

  The fields here are:

  a) address is the address range, in the process’ address space, that the mapping occupies. This part was already treated by GDB, so I did not have to worry about it.

  b) perms is a set of permissions (r ead, w rite, e x ecute, s hared, p rivate [COW – copy-on-write]) applied to the memory mapping. GDB was already dealing with rwx permissions, but I needed to include the p flag as well. I also made GDB ignore the mappings that did not have the r flag active, because it does not make sense to dump something that you cannot read.

  c) offset is the offset into the applied to the file, if the mapping is file-backed (see below). GDB already handled this correctly.

  d) dev is the device (major:minor) related to the file, if there is one. GDB already handled this correctly, though I was using this field for more things (continue reading).

  e) inode is the inode on the device above. The value of zero means that no inode is associated with the memory mapping. Nothing to do here.

  f) pathname is the file associate with this mapping, if there is one. This is one of the most important fields that I had to use, and one of the most complicated to understand completely. GDB now uses this to heuristically identify whether the mapping is anonymous or not.

• GDB is now also interested in Anonymous: and AnonHugePages: fields from the smaps file. Those fields represent the content of anonymous data on the mapping; if GDB finds that this content is greater than zero, this means that the mapping is anonymous.

• The last, but perhaps most important field, is the VmFlags: field. It contains a series of two-letter flags that provide very useful information about the mapping. A description of the fields is: a) sh: the mapping is shared (VM_SHARED) b) dd: this mapping should not be dumped in a corefile (VM_DONTDUMP) c) ht: this is HugeTLB mapping

With that in hands, the following task was to be able to determine whether a memory mapping is anonymous or file-backed, private or shared.

Types of memory mappings

There can be four types of memory mappings:

1. Anonymous private mapping
2. Anonymous shared mapping
3. File-backed private mapping
4. File-backed shared mapping

It should be possible to uniquely identify each mapping based on the information provided by the smaps file; however, you will see that this is not always the case. Below, I will explain how to determine each of the four characteristics that define a mapping.

Anonymous

A mapping is anonymous if one of these conditions apply:

1. The pathname associated with it is either /dev/zero (deleted), /SYSV%08x (deleted), or <filename> (deleted) (see below).
2. There is content in the Anonymous: or in the AnonHugePages: fields of the mapping in the smaps file.

A special explanation is needed for the <filename> (deleted) case. It is not always guaranteed that it identifies an anonymous mapping; in fact, it is possible to have the (deleted) part for file-backed mappings as well (say, when you are running a program that uses shared libraries, and those shared libraries have been removed because of an update, for example). However, we are trying to mimic the behavior of the Linux kernel here, which checks to see if a file has no hard links associated with it (and therefore is truly deleted).

Although it may be possible for the userspace to do an extensive check (by stat ing the file, for example), the Linux kernel certainly could give more information about this.

File-backed

A mapping is file-backed (i.e., not anonymous) if:

1. The pathname associated with it contains a <filename>, without the (deleted) part.

As has been explained above, a mapping whose pathname contains the (deleted) string could still be file-backed, but we decide to consider it anonymous.

It is also worth mentioning that a mapping can be simultaneously anonymous and file-backed: this happens when the mapping contains a valid pathname (without the (deleted) part), but also contains Anonymous: or AnonHugePages: contents.

Private

A mapping is considered to be private (i.e., not shared) if:

1. In the absence of the VmFlags field (in the smaps file), its permission field has the flag p.
2. If the VmFlags field is present, then the mapping is private if we do not find the sh flag there.

Shared

A mapping is shared (i.e., not private) if:

1. In the absence of VmFlags in the smaps file, the permission field of the mapping does not have the p flag. Not having this flag actually means VM_MAYSHARE and not necessarily VM_SHARED (which is what we want), but it is the best approximation we have.
2. If the VmFlags field is present, then the mapping is shared if we find the sh flag there.

The patch

With all that in mind, I hacked GDB to improve the coredump mechanism for GNU/Linux operating systems. The main function which decides the memory mappings that will or will not be dumped on GNU/Linux is linux_find_memory_regions_full; the Linux kernel obviously uses its own function, vma_dump_size, to do the same thing.

Linux has one advantage: it is a kernel, and therefore has much more knowledge about processes’ internals than a userspace program. For example, inside Linux it is trivial to check if a file marked as “(deleted)” in the output of the smaps file has no hard links associated with it (and therefore is not really deleted); the same operation on userspace, however, would require root access to inspect the contents of the /proc/<PID>/map_files/ directory.

The case described above, if you remember, is something that impacts the ability to tell whether a mapping is anonymous or not. I am talking to the Linux kernel guys to see if it is possible to export this information directly via the smaps file, instead of having to do the current heuristic.

While doing this work, some strange behaviors were found in the Linux kernel. Oleg is working on them, along with other Linux hackers. From our side, there is still room for improvement on this code. The first thing I can think of is to improve the heuristics for finding anonymous mappings. Another relatively easy thing to do would be to let the user specify a value for coredump_filter on the command line, without editing the /proc file. And of course, keep this code always updated with its counterpart in the Linux kernel.

Upstream discussions and commit

If you are interested, you can see the discussions that happened upstream by going to this link. This is the fourth (and final) submission of the patch; you should be able to find the other submissions in the archive.

The final commit can be found in the official repository.