Messages in this thread | | | Date | Thu, 26 Oct 2023 04:57:42 +0530 | Subject | Re: [PATCH] Fixing warning of directly dereferencing __rcu tagged | From | Abhinav Singh <> |
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On 10/26/23 04:08, Andrew Morton wrote: > On Thu, 26 Oct 2023 03:58:11 +0530 Abhinav Singh <singhabhinav9051571833@gmail.com> wrote: > >> This patch fixes the warning about directly dereferencing a pointer >> tagged with __rcu annotation. >> >> Dereferencing the pointers tagged with __rcu directly should >> always be avoided according to the docs. There is a rcu helper >> functions rcu_dereference(...) to use when dereferencing a __rcu >> pointer. This functions returns the non __rcu tagged pointer. > > Seems sensible. > >> Like normal pointer there should be a check for null case when >> further dereferencing the returned dereferenced __rcu pointer. > > Why is this? > >> --- a/kernel/fork.c >> +++ b/kernel/fork.c >> @@ -2369,7 +2369,9 @@ __latent_entropy struct task_struct *copy_process( >> >> retval = -EAGAIN; >> if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) { >> - if (p->real_cred->user != INIT_USER && >> + const struct cred *real_cred = rcu_dereference(p->real_cred); >> + >> + if (real_cred && real_cred->user != INIT_USER && >> !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) >> goto bad_fork_cleanup_count; > > The old code assumes that p->read_cred cannot be NULL and the new code > does nothing to make it possible that `real_cred' can be NULL? > > In other words, I see no reason to add this new check for NULL?
Thank you for the response!
I thought it will be better to have check before accessing it, just so we dont have any segmentation fault in future.
Also I just noticed there are two more places where direct dereferencing of __rcu pointer is done in this same file. Should I do those changes in this patch ?
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