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 was requested
needed to respect the
/proc/<PID>/coredump_filter file when generating
a coredump (i.e., when you use the
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
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
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
rwxpermissions, but I needed to include the
pflag as well. I also made GDB ignore the mappings that did not have the
rflag 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
AnonHugePages:fields from the
smapsfile. 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 (
dd: this mapping should not be dumped in a corefile (
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:
- Anonymous private mapping
- Anonymous shared mapping
- File-backed private mapping
- 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.
A mapping is anonymous if one of these conditions apply:
pathnameassociated with it is either
/SYSV%08x (deleted), or
<filename> (deleted)(see below).
- There is content in the
Anonymous:or in the
AnonHugePages:fields of the mapping in the
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
stat ing the file, for example), the Linux kernel certainly could
give more information about this.
A mapping is file-backed (i.e., not anonymous) if:
pathnameassociated with it contains a
<filename>, without the
As has been explained above, a mapping whose
pathname contains the
(deleted) string could still be file-backed, but we decide to consider
It is also worth mentioning that a mapping can be simultaneously
anonymous and file-backed: this happens when the mapping contains a
pathname (without the
(deleted) part), but also contains
A mapping is considered to be private (i.e., not shared) if:
- In the absence of the
VmFlagsfield (in the
smapsfile), its permission field has the flag
- If the
VmFlagsfield is present, then the mapping is private if we do not find the
A mapping is shared (i.e., not private) if:
- In the absence of
smapsfile, the permission field of the mapping does not have the
pflag. Not having this flag actually means
VM_MAYSHAREand not necessarily
VM_SHARED(which is what we want), but it is the best approximation we have.
- If the
VmFlagsfield is present, then the mapping is shared if we find the
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
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
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
/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.