[CRIU] [PATCH 3/7] Skip adding PROT_READ to non-PROT_READ mappings

[Abhishek Dubey]

"read" mode pre-dump may fail even after
adding PROT_READ flag. Adding PROT_READ
works when dumping statically. See added
comment for details.

Signed-off-by: Abhishek Dubey <dubeyabhishek777 at gmail.com>
---
 criu/mem.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 48 insertions(+), 13 deletions(-)

diff --git a/criu/mem.c b/criu/mem.c
index 640b00a..15094f7 100644
--- a/criu/mem.c
+++ b/criu/mem.c
@@ -553,18 +553,51 @@ int parasite_dump_pages_seized(struct pstree_item *item,
 	 * able to read the memory contents.
 	 *
 	 * Afterwards -- reprotect memory back.
+	 *
+	 * This step is required for "splice" mode pre-dump and dump.
+	 * Skip this step for "read" mode pre-dump.
+	 * "read" mode pre-dump delegates processing of non-PROT_READ
+	 * regions to dump stage. Adding PROT_READ works fine for
+	 * static processing and fails for dynamic as explained below.
+	 *
+	 * Consider following sequence of instances to reason, why
+	 * not to add PROT_READ in "read" mode pre-dump ?
+	 *
+	 *    CRIU- "read" pre-dump         Target Process
+	 *
+	 *                              1. Creates mapping M
+	 *                                 without PROT_READ
+	 * 2. CRIU freezes target
+	 *    process
+	 * 3. Collect the mappings
+	 * 4. Add PROT_READ to M
+	 *    (non-PROT_READ region)
+	 * 5. CRIU unfreezes target
+	 *    process
+	 *                              6. Add flag PROT_READ
+	 *                                 to mapping M
+	 *                              7. Revoke flag PROT_READ
+	 *                                 from mapping M
+	 * 8. process_vm_readv tries
+	 *    to copy mapping M
+	 *    (believing M have
+	 *     PROT_READ flag)
+	 * 9. syscall fails to copy
+	 *    data from M
 	 */
 
-	pargs->add_prot = PROT_READ;
-	ret = compel_rpc_call_sync(PARASITE_CMD_MPROTECT_VMAS, ctl);
-	if (ret) {
-		pr_err("Can't dump unprotect vmas with parasite\n");
-		return ret;
-	}
+	if (!mdc->pre_dump || opts.pre_dump_mode == PRE_DUMP_SPLICE) {
+		pargs->add_prot = PROT_READ;
+		ret = compel_rpc_call_sync(PARASITE_CMD_MPROTECT_VMAS, ctl);
+		if (ret) {
+			pr_err("Can't dump unprotect vmas with parasite\n");
+			return ret;
+		}
 
-	if (fault_injected(FI_DUMP_PAGES)) {
-		pr_err("fault: Dump VMA pages failure!\n");
-		return -1;
+		if (fault_injected(FI_DUMP_PAGES)) {
+			pr_err("fault: Dump VMA pages failure!\n");
+			return -1;
+		}
 	}
 
 	ret = __parasite_dump_pages_seized(item, pargs, vma_area_list, mdc, ctl);
@@ -574,10 +607,12 @@ int parasite_dump_pages_seized(struct pstree_item *item,
 		return ret;
 	}
 
-	pargs->add_prot = 0;
-	if (compel_rpc_call_sync(PARASITE_CMD_MPROTECT_VMAS, ctl)) {
-		pr_err("Can't rollback unprotected vmas with parasite\n");
-		ret = -1;
+	if (!mdc->pre_dump || opts.pre_dump_mode == PRE_DUMP_SPLICE) {
+		pargs->add_prot = 0;
+		if (compel_rpc_call_sync(PARASITE_CMD_MPROTECT_VMAS, ctl)) {
+			pr_err("Can't rollback unprotected vmas with parasite\n");
+			ret = -1;
+		}
 	}
 
 	return ret;
-- 
2.7.4