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Tue, 05 May 2020 07:17:59 +0000 Received: from b03ledav005.gho.boulder.ibm.com (b03ledav005.gho.boulder.ibm.com [9.17.130.236]) by b03cxnp08025.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 0457HwjU22610176 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 5 May 2020 07:17:58 GMT Received: from b03ledav005.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7CEC7BE04F; Tue, 5 May 2020 07:17:58 +0000 (GMT) Received: from b03ledav005.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3E71CBE051; Tue, 5 May 2020 07:17:56 +0000 (GMT) Received: from skywalker.ibmuc.com (unknown [9.85.81.70]) by b03ledav005.gho.boulder.ibm.com (Postfix) with ESMTP; Tue, 5 May 2020 07:17:55 +0000 (GMT) From: "Aneesh Kumar K.V" To: linuxppc-dev@lists.ozlabs.org, mpe@ellerman.id.au Subject: [PATCH v4 07/22] powerpc/perf/callchain: Use __get_user_pages_fast in read_user_stack_slow Date: Tue, 5 May 2020 12:47:14 +0530 Message-Id: <20200505071729.54912-8-aneesh.kumar@linux.ibm.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200505071729.54912-1-aneesh.kumar@linux.ibm.com> References: <20200505071729.54912-1-aneesh.kumar@linux.ibm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138, 18.0.676 definitions=2020-05-05_02:2020-05-04, 2020-05-05 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 malwarescore=0 suspectscore=0 bulkscore=0 priorityscore=1501 spamscore=0 lowpriorityscore=0 adultscore=0 phishscore=0 mlxlogscore=999 clxscore=1015 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2003020000 definitions=main-2005050053 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Aneesh Kumar K.V" , npiggin@gmail.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" read_user_stack_slow is called with interrupts soft disabled and it copies contents from the page which we find mapped to a specific address. To convert userspace address to pfn, the kernel now uses lockless page table walk. The kernel needs to make sure the pfn value read remains stable and is not released and reused for another process while the contents are read from the page. This can only be achieved by holding a page reference. One of the first approaches I tried was to check the pte value after the kernel copies the contents from the page. But as shown below we can still get it wrong CPU0 CPU1 pte = READ_ONCE(*ptep); pte_clear(pte); put_page(page); page = alloc_page(); memcpy(page_address(page), "secret password", nr); memcpy(buf, kaddr + offset, nb); put_page(page); handle_mm_fault() page = alloc_page(); set_pte(pte, page); if (pte_val(pte) != pte_val(*ptep)) Hence switch to __get_user_pages_fast. Signed-off-by: Aneesh Kumar K.V --- arch/powerpc/perf/callchain_64.c | 46 ++++++++++---------------------- 1 file changed, 14 insertions(+), 32 deletions(-) diff --git a/arch/powerpc/perf/callchain_64.c b/arch/powerpc/perf/callchain_64.c index df1ffd8b20f2..b63086b663ef 100644 --- a/arch/powerpc/perf/callchain_64.c +++ b/arch/powerpc/perf/callchain_64.c @@ -26,43 +26,25 @@ */ int read_user_stack_slow(void __user *ptr, void *buf, int nb) { - int ret = -EFAULT; - pgd_t *pgdir; - pte_t *ptep, pte; - unsigned int shift; + unsigned long addr = (unsigned long) ptr; unsigned long offset; - unsigned long pfn, flags; + struct page *page; + int nrpages; void *kaddr; - pgdir = current->mm->pgd; - if (!pgdir) - return -EFAULT; + nrpages = __get_user_pages_fast(addr, 1, 1, &page); + if (nrpages == 1) { + kaddr = page_address(page); + + /* align address to page boundary */ + offset = addr & ~PAGE_MASK; - local_irq_save(flags); - ptep = find_current_mm_pte(pgdir, addr, NULL, &shift); - if (!ptep) - goto err_out; - if (!shift) - shift = PAGE_SHIFT; - - /* align address to page boundary */ - offset = addr & ((1UL << shift) - 1); - - pte = READ_ONCE(*ptep); - if (!pte_present(pte) || !pte_user(pte)) - goto err_out; - pfn = pte_pfn(pte); - if (!page_is_ram(pfn)) - goto err_out; - - /* no highmem to worry about here */ - kaddr = pfn_to_kaddr(pfn); - memcpy(buf, kaddr + offset, nb); - ret = 0; -err_out: - local_irq_restore(flags); - return ret; + memcpy(buf, kaddr + offset, nb); + put_page(page); + return 0; + } + return -EFAULT; } static int read_user_stack_64(unsigned long __user *ptr, unsigned long *ret) -- 2.26.2