linux-kernel - [PATCH] mm: vmap: avoid -Wsequence-point warning

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Message-Id: <20211104133549.1150058-1-arnd@kernel.org>
Date:   Thu,  4 Nov 2021 14:35:40 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Chen Wandun <chenwandun@...wei.com>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: vmap: avoid -Wsequence-point warning

From: Arnd Bergmann <arnd@...db.de>

gcc warns about potentially undefined behavior in an array index:

mm/vmalloc.c: In function 'vmap_pfn_apply':
mm/vmalloc.c:2800:58: error: operation on 'data->idx' may be undefined [-Werror=sequence-point]
 2800 |         *pte = pte_mkspecial(pfn_pte(data->pfns[data->idx++], data->prot));
      |                                                 ~~~~~~~~~^~
arch/arm64/include/asm/pgtable-types.h:25:37: note: in definition of macro '__pte'
   25 | #define __pte(x)        ((pte_t) { (x) } )
      |                                     ^
arch/arm64/include/asm/pgtable.h:80:15: note: in expansion of macro '__phys_to_pte_val'
   80 |         __pte(__phys_to_pte_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
      |               ^~~~~~~~~~~~~~~~~
mm/vmalloc.c:2800:30: note: in expansion of macro 'pfn_pte'
 2800 |         *pte = pte_mkspecial(pfn_pte(data->pfns[data->idx++], data->prot));
      |                              ^~~~~~~

This only appeared in one randconfig build so far, and I don't know
what caused it, but moving the index increment out of the expression
at least addresses the warning.

Fixes: 3e9a9e256b1e ("mm: add a vmap_pfn function")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
Not sure if it's worth doing a deeper analysis of how the compiler
thinks this might go wrong, it would appear that it thinks 'pte'
might be an alias for 'data' here, but only in some configurations.

If you want to try reproducing it, the .config that triggered it is
https://pastebin.com/caeKD1Wv
---
 mm/vmalloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d2a00ad4e1dd..cdac02aab6b1 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2797,7 +2797,9 @@ static int vmap_pfn_apply(pte_t *pte, unsigned long addr, void *private)
 
 	if (WARN_ON_ONCE(pfn_valid(data->pfns[data->idx])))
 		return -EINVAL;
-	*pte = pte_mkspecial(pfn_pte(data->pfns[data->idx++], data->prot));
+	*pte = pte_mkspecial(pfn_pte(data->pfns[data->idx], data->prot));
+	data->idx++;
+
 	return 0;
 }
 
-- 
2.29.2