Messages in this thread | | | From | Nicolai Stange <> | Subject | Re: [kbuild-all] [PATCH] OPTIONAL: cpufreq/intel_pstate: fix debugfs_simple_attr.cocci warnings | Date | Fri, 30 Mar 2018 08:14:40 +0200 |
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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,
Nicolai
-- SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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