[PATCH v4 3/5] selftests/mm: fail KSM helpers on short reads and writes

[Qiliang Yuan]

The KSM helpers in vm_util read and write KSM's sysfs and procfs files
with a single pread() or write(), and return -errno when the call does
not transfer what they asked for.

A read of zero bytes or a short write leaves errno at 0, so the helpers
then return 0, which their callers take as a value of 0 or as a write
that took effect. A short write of run makes ksm_start() wait for scans
of a ksmd that never started, a short read of full_scans makes it wait
for scans that never seem to complete, and short reads of
ksm_merging_pages or ksm_stat and a short write of use_zero_pages make
cases fail that should have been skipped.

Return -errno only when the call fails, and -EIO when it transfers less
than asked, so that callers skip or fail as they do on other errors.
Forcing each short read or write with an LD_PRELOAD shim on
ksm_functional_tests:

  short read or write of   before                    after
  run                      cases fail, then hangs    all skipped
  full_scans               hangs in the first case   all skipped
  ksm_merging_pages        9 of 11 cases fail        all skipped
  ksm_stat                 zero page case fails      zero page case skipped
  use_zero_pages           zero page case fails      zero page case skipped

Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
---
The checks come from 93fb70aa5904 ("selftests/vm: add KSM unmerge
tests"), 3d0745e59c84 ("selftest: add a testcase of ksm zero pages")
and 42096aa24b82 ("selftest/mm: ksm_functional_tests: test in
mmap_and_merge_range() if anything got merged"), and b27f292de6b1
("selftests/mm: put general ksm operation into vm_util") turned the
use_zero_pages check into one of them. There are no Fixes tags, as
only short reads and writes forced with the shim hit them, and the
helpers are in vm_util only since v6.18.
---
 tools/testing/selftests/mm/vm_util.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index 515031b8283da..caea045fa7d46 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -920,6 +920,8 @@ long ksm_get_self_zero_pages(void)
 	close(proc_self_ksm_stat_fd);
 	if (read_size < 0)
 		return -errno;
+	if (!read_size)
+		return -EIO;
 
 	buf[read_size] = 0;
 
@@ -944,8 +946,10 @@ long ksm_get_self_merging_pages(void)
 
 	ret = pread(proc_self_ksm_merging_pages_fd, buf, sizeof(buf) - 1, 0);
 	close(proc_self_ksm_merging_pages_fd);
-	if (ret <= 0)
+	if (ret < 0)
 		return -errno;
+	if (!ret)
+		return -EIO;
 	buf[ret] = 0;
 
 	return strtol(buf, NULL, 10);
@@ -963,8 +967,10 @@ long ksm_get_full_scans(void)
 
 	ret = pread(ksm_full_scans_fd, buf, sizeof(buf) - 1, 0);
 	close(ksm_full_scans_fd);
-	if (ret <= 0)
+	if (ret < 0)
 		return -errno;
+	if (!ret)
+		return -EIO;
 	buf[ret] = 0;
 
 	return strtol(buf, NULL, 10);
@@ -981,7 +987,9 @@ int ksm_use_zero_pages(void)
 
 	ret = write(ksm_use_zero_pages_fd, "1", 1);
 	close(ksm_use_zero_pages_fd);
-	return ret == 1 ? 0 : -errno;
+	if (ret < 0)
+		return -errno;
+	return ret == 1 ? 0 : -EIO;
 }
 
 int ksm_start(void)
@@ -1002,8 +1010,10 @@ int ksm_start(void)
 	}
 	ret = write(ksm_fd, "1", 1);
 	close(ksm_fd);
-	if (ret != 1)
+	if (ret < 0)
 		return -errno;
+	if (ret != 1)
+		return -EIO;
 	do {
 		end_scans = ksm_get_full_scans();
 		if (end_scans < 0)
@@ -1024,7 +1034,9 @@ int ksm_stop(void)
 
 	ret = write(ksm_fd, "2", 1);
 	close(ksm_fd);
-	return ret == 1 ? 0 : -errno;
+	if (ret < 0)
+		return -errno;
+	return ret == 1 ? 0 : -EIO;
 }
 
 int get_hardware_corrupted_size(unsigned long *val)

-- 
2.43.0