Messages in this thread | | | From | Nicolai Stange <> | Subject | Re: [kbuild-all] [PATCH] OPTIONAL: cpufreq/intel_pstate: fix debugfs_simple_attr.cocci warnings | Date | Sat, 31 Mar 2018 06:20:32 +0200 |
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Julia Lawall <julia.lawall@lip6.fr> writes:
> On Fri, 30 Mar 2018, Nicolai Stange wrote: > >> Julia Lawall <julia.lawall@lip6.fr> writes: >> >> > On Thu, 29 Mar 2018, Fabio Estevam wrote: >> > >> >> Hi Julia, >> >> >> >> On Thu, Mar 29, 2018 at 4:12 PM, Julia Lawall <julia.lawall@lip6.fr> wrote: >> >> > Use DEFINE_DEBUGFS_ATTRIBUTE rather than DEFINE_SIMPLE_ATTRIBUTE >> >> > for debugfs files. >> >> > >> >> > Semantic patch information: >> >> > Rationale: DEFINE_SIMPLE_ATTRIBUTE + debugfs_create_file() >> >> > imposes some significant overhead as compared to >> >> > DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe(). >> >> >> >> Just curious: could you please expand on what "imposes some >> >> significant overhead" means? >> > >> > I don't know. I didn't write this rule. Nicolai, can you explain? >> >> From commit 49d200deaa68 ("debugfs: prevent access to removed files' private >> data"): >> >> Upon return of debugfs_remove()/debugfs_remove_recursive(), it might >> still be attempted to access associated private file data through >> previously opened struct file objects. If that data has been freed by >> the caller of debugfs_remove*() in the meanwhile, the reading/writing >> process would either encounter a fault or, if the memory address in >> question has been reassigned again, unrelated data structures could get >> overwritten. >> [...] >> Currently, there are ~1000 call sites of debugfs_create_file() spread >> throughout the whole tree and touching all of those struct file_operations >> in order to make them file removal aware by means of checking the result of >> debugfs_use_file_start() from within their methods is unfeasible. >> >> Instead, wrap the struct file_operations by a lifetime managing proxy at >> file open [...] >> >> The additional overhead comes in terms of additional memory needed: for >> debugs files created through debugfs_create_file(), one such struct >> file_operations proxy is allocated for each struct file instantiation, >> c.f. full_proxy_open(). >> >> This was needed to "repair" the ~1000 call sites without touching them. >> >> New debugfs users should make their file_operations removal aware >> themselves by means of DEFINE_DEBUGFS_ATTRIBUTE() and signal that fact to >> the debugfs core by instantiating them through >> debugfs_create_file_unsafe(). >> >> See commit c64688081490 ("debugfs: add support for self-protecting >> attribute file fops") for further information. > > Thanks. Perhaps it would be good to add a reference to this commit in > the message generated by the semantic patch.
Thanks for doing this!
> > Would it be sufficient to just apply the semantic patch everywhere and > submit the patches?
In principle yes. But I'm note sure whether such a mass application is worth it: the proxy allocation happens only at file open and the expectation is that there aren't that many debugfs files kept open at a time. OTOH, a struct file_operation consumes 256 bytes with sizeof(long) == 8.
In my opinion, new users should avoid this overhead as it's easily doable. For existing ones, I don't know.
Thanks,
Nicolai
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