Crash command: vm

[qiyeliu]

https://crash-utility.github.io/help_pages/vm.html

Hi @osandov,
I would like to contribute by implementing a vm command (for virtual memory inspection) for the crash module. Before I proceed, I'd appreciate any high-level pointers or architectural advice you might have.

[osandov]

Great, thank you! I'd find it easiest to review an initial version with support for no options and maybe the pid|task option. I think we already have all of the helpers needed to implement that.

I can't tell what -R is supposed to do. The examples in crash show matching based on vm_flags and virtual address, but the help string mentions filenames.

Most of the other options look straightforward and could be done in followup PRs, with the following exceptions:

• -f is a bit annoying because the flags used in vm_flags are macros and would need to be hard-coded. It'd be nice to send a kernel patch upstream to convert those to enums.
• For -p/-P, I need to figure out how I want to generalize page table lookups for a few commands before we do anything for this one.

[osandov]

Now that the basic command is implemented, do you plan to work on any of the options, or should I create a followup for later? Either way is fine with me. -m, -v, -x, and -d should be very simple; _crash_command_task() is a good example for those. The other flags are low-priority as we discussed before.

[qiyeliu]

Now that the basic command is implemented, do you plan to work on any of the options, or should I create a followup for later? Either way is fine with me. -m, -v, -x, and -d should be very simple; _crash_command_task() is a good example for those. The other flags are low-priority as we discussed before.

I'll work on implementing the -m, -v, -x, and -d options next.
Regarding vm_flags, I submitted a patch, but it wasn't accepted. Lorenzo Stoakes indicated he would work on it; see the link below.（https://lore.kernel.org/all/20251011093054.886460-1-ye.liu@linux.dev/）

[osandov]

It looks like Lorenzo did end up adding an enum for the bit numbers for 6.19: torvalds/linux@2b6a3f0. So we can do something like:

from drgn import ObjectNotFoundError

# Since Linux kernel commit 2b6a3f061f11 ("mm: declare VMA flags by bit") (in
# v6.19), VMA flag bit numbers are defined in an anonymous enum. Before that,
# we have to hard-code them.
try:
    for name, bit in prog["VMA_READ_BIT"].type_.enumerators:
        ...
except ObjectNotFoundError:
    # Use hard-coded list...