Re: [PATCH] Fixing warning of directly dereferencing __rcu tagged

[Abhinav Singh <singhabhinav9051571833@gmail.com>]

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On 10/26/23 05:20, Andrew Morton wrote:
> On Thu, 26 Oct 2023 04:57:42 +0530 Abhinav Singh <singhabhinav9051571833@gmail.com> wrote:
> 
>> On 10/26/23 04:08, Andrew Morton wrote:
>>>> +++ 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.
> 
> That would be adding code which has no effect?
> 
>> 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 ?
> 
> I don't see why.  rcu_dereference(p) cannot return NULL if `p' is non-NULL?


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From 598bf82b611a9af96c5d412855957e378523f529 Mon Sep 17 00:00:00 2001
From: Abhinav Singh <singhabhinav9051571833@gmail.com>
Date: Tue, 24 Oct 2023 15:24:52 +0530
Subject: [PATCH] Fixing warning cast removes address space '__iomem' of
 expression

This patch fixes sparse complaining about the removal of __iomem address
space when casting the return value of this function ioremap_cache(...)
from `void __ioremap*` to `void*`.

I think there are two way of fixing it, first one is changing the
datatype of variable `ghcb_va` from `void*` to `void __iomem*` . 
Second way of fixing it is using the memremap(...) which is 
done in this patch.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
---
 arch/x86/hyperv/hv_init.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 21556ad87f4b..c14161add274 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -70,7 +70,7 @@ static int hyperv_init_ghcb(void)
 
 	/* Mask out vTOM bit. ioremap_cache() maps decrypted */
 	ghcb_gpa &= ~ms_hyperv.shared_gpa_boundary;
-	ghcb_va = (void *)ioremap_cache(ghcb_gpa, HV_HYP_PAGE_SIZE);
+	ghcb_va = memremap(ghcb_gpa, HV_HYP_PAGE_SIZE, MEMREMAP_WB);
 	if (!ghcb_va)
 		return -ENOMEM;
 
-- 
2.39.2