shmem: Check if we can seek over VMA holes first

Using seek() to skip past holes in VM may not be possible. Check if it's
supported before attempting to do it.

Co-authored-by: Ivanq <[email protected]>
Signed-off-by: Younes Manton <[email protected]>

criu/shmem.c

@@ -724,7 +724,7 @@ static int next_data_segment(int fd, unsigned long pfn, unsigned long *next_data
 	return 0;
 }
 
-static int do_dump_one_shmem(int fd, void *addr, struct shmem_info *si)
+static int do_dump_one_shmem(int fd, void *addr, struct shmem_info *si, bool seek_data_supported)
 {
 	struct page_pipe *pp;
 	struct page_xfer xfer;
@@ -750,7 +750,8 @@ static int do_dump_one_shmem(int fd, void *addr, struct shmem_info *si)
 		unsigned long pgaddr;
 		int st = -1;
 
-		if (pfn >= next_hole_pfn && next_data_segment(fd, pfn, &next_data_pnf, &next_hole_pfn))
+		if (seek_data_supported && pfn >= next_hole_pfn &&
+		    next_data_segment(fd, pfn, &next_data_pnf, &next_hole_pfn))
 			goto err_xfer;
 
 		if (si->pstate_map && is_shmem_tracking_en()) {
@@ -808,20 +809,49 @@ static int dump_one_shmem(struct shmem_info *si)
 {
 	int fd, ret = -1;
 	void *addr;
+	bool seek_data_supported;
 
 	pr_info("Dumping shared memory %ld\n", si->shmid);
 
-	fd = open_proc(si->pid, "map_files/%lx-%lx", si->start, si->end);
-	if (fd < 0)
-		goto err;
+	fd = __open_proc(si->pid, EPERM, O_RDONLY, "map_files/%lx-%lx", si->start, si->end);
+	if (fd >= 0) {
+		addr = mmap(NULL, si->size, PROT_READ, MAP_SHARED, fd, 0);
+		if (addr == MAP_FAILED) {
+			pr_err("Can't map shmem 0x%lx (0x%lx-0x%lx)\n", si->shmid, si->start, si->end);
+			goto errc;
+		}
 
-	addr = mmap(NULL, si->size, PROT_READ, MAP_SHARED, fd, 0);
-	if (addr == MAP_FAILED) {
-		pr_err("Can't map shmem 0x%lx (0x%lx-0x%lx)\n", si->shmid, si->start, si->end);
-		goto errc;
+		seek_data_supported = true;
+	} else {
+		if (errno != EPERM) {
+			goto err;
+		}
+
+		fd = open_proc(si->pid, "mem");
+		if (fd < 0) {
+			goto err;
+		}
+
+		addr = mmap(NULL, si->size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+		if (addr == MAP_FAILED) {
+			pr_err("Can't map empty space for shmem 0x%lx (0x%lx-0x%lx)\n", si->shmid, si->start, si->end);
+			goto errc;
+		}
+
+		if (lseek(fd, si->start, SEEK_SET) < 0) {
+			pr_perror("Can't seek virtual memory");
+			return -1;
+		}
+
+		if (read(fd, addr, si->size) < si->size) {
+			pr_perror("Can't read virtual memory");
+			return -1;
+		}
+
+		seek_data_supported = false;
 	}
 
-	ret = do_dump_one_shmem(fd, addr, si);
+	ret = do_dump_one_shmem(fd, addr, si, seek_data_supported);
 
 	munmap(addr, si->size);
 errc:
@@ -849,7 +879,7 @@ int dump_one_memfd_shmem(int fd, unsigned long shmid, unsigned long size)
 		goto err;
 	}
 
-	ret = do_dump_one_shmem(fd, addr, &si);
+	ret = do_dump_one_shmem(fd, addr, &si, true);
 
 	munmap(addr, size);
 err:
@@ -875,7 +905,7 @@ int dump_one_sysv_shmem(void *addr, unsigned long size, unsigned long shmid)
 	if (fd < 0)
 		return -1;
 
-	ret = do_dump_one_shmem(fd, addr, si);
+	ret = do_dump_one_shmem(fd, addr, si, true);
 	close(fd);
 	return ret;
 }