On Thu, Oct 7, 2021 at 12:03 PM 'John Hubbard' via kernel-team
[off-list ref] wrote:

On 10/7/21 11:50, Suren Baghdasaryan wrote:
...

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I believe Pavel meant something as simple as
$ YOUR_FILE=$YOUR_IDS_DIR/my_string_name
$ touch $YOUR_FILE
$ stat -c %i $YOUR_FILE

Ah, ok, now I understand the proposal. Thanks for the clarification!
So, this would use filesystem as a directory for inode->name mappings.
One rough edge for me is that the consumer would still need to parse
/proc/$pid/maps and convert [anon:inode] into [anon:name] instead of
just dumping the content for the user. Would it be acceptable if we
require the ID provided by prctl() to always be a valid inode and
show_map_vma() would do the inode-to-filename conversion when
generating maps/smaps files? I know that inode->dentry is not
one-to-one mapping but we can simply output the first dentry name.
WDYT?

No. You do not want to dictate any particular way of the mapping. The
above is just one way to do that without developing any actual mapping
yourself. You just use a filesystem for that. Kernel doesn't and
shouldn't understand the meaning of those numbers. It has no business in
that.

In a way this would be pushing policy into the kernel.

I can see your point. Any other ideas on how to prevent tools from
doing this id-to-name conversion themselves?

I really fail to understand why you really want to prevent them from that.
Really, the whole thing is just a cookie that kernel maintains for memory
mappings so that two parties can understand what the meaning of that
mapping is from a higher level. They both have to agree on the naming
but the kernel shouldn't dictate any specific convention because the
kernel _doesn't_ _care_. These things are not really anything actionable
for the kernel. It is just a metadata.

The desire is for one of these two parties to be a human who can get
the data and use it as is without additional conversions.
/proc/$pid/maps could report FD numbers instead of pathnames, which
could be converted to pathnames in userspace. However we do not do
that because pathnames are more convenient for humans to identify a
specific resource. Same logic applies here IMHO.

Yes, please. It really seems like the folks that are interested in this
feature want strings. (I certainly do.) For those not interested in the
feature, it sounds like a CONFIG to keep it away would be sufficient.
Can we just move forward with that?

Would love to if others are ok with this.

If this doesn't get accepted, then another way forward would to continue
the ideas above to their logical conclusion, and create a new file system:
vma-fs.  Like debug-fs and other special file systems, similar policy and
motivation. Also protected by a CONFIG option.

TBH, I would prefer to have the current simple solution protected with
a CONFIG option.

Actually this seems at least as natural as the procfs approach, especially
given the nature of these strings, which feel more like dir+file names, than
simple strings.

thanks,
--
John Hubbard
NVIDIA

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