[PATCH] arm64: mmu: no write cache for O_SYNC flag

Li Wang
2020-03-26T16:38:08Z

reproduce steps:
1.
disable CONFIG_STRICT_DEVMEM in linux kernel
2.
Process A gets a Physical Address of global variable by
"/proc/self/pagemap".
3.
Process B writes a value to the same Physical Address by mmap():
fd=open("/dev/mem",O_SYNC);
Virtual Address=mmap(fd);

problem symptom:
after Process B write a value to the Physical Address,
Process A of the value of global variable does not change.
They both W/R the same Physical Address.

technical reason:
Process B writing the Physical Address is by the Virtual Address,
and the Virtual Address comes from "/dev/mem" and mmap().
In arm64 arch, the Virtual Address has write cache.
So, maybe the value is not written into Physical Address.

fix reason:
giving write cache flag in arm64 is in phys_mem_access_prot():
=====
arch/arm64/mm/mmu.c
phys_mem_access_prot()
{
if (!pfn_valid(pfn))
return pgprot_noncached(vma_prot);
else if (file->f_flags & O_SYNC)
return pgprot_writecombine(vma_prot);
return vma_prot;
}
====
the other arch and the share function drivers/char/mem.c of phys_mem_access_prot()
does not add write cache flag.
So, removing the flag to fix the issue

Signed-off-by: Li Wang <li.wang@windriver.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
---
arch/arm64/mm/mmu.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 128f70852bf3..d7083965ca17 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -81,8 +81,6 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
{
if (!pfn_valid(pfn))
return pgprot_noncached(vma_prot);
- else if (file->f_flags & O_SYNC)
- return pgprot_writecombine(vma_prot);
return vma_prot;
}
EXPORT_SYMBOL(phys_mem_access_prot);
--
2.24.1