selftests/cgroup: read the entire file in read_text()

read_text() issues a single read(2) and returns whatever it got, so a
caller that asks for a whole file silently gets a prefix of it instead.
For a seq_file larger than one page it never gets more than that prefix:
seq_read_iter() copies out at most one internal PAGE_SIZE buffer per call
and leaves the rest for the next read(2), no matter how much room the
caller's buffer has.

/proc/self/mounts is one of those files, so cg_find_root() only ever sees
the first 4K of the mount table.  On a machine with ~80 mounts the cgroup2
entry already sits past that limit: the file is 7177 bytes, the single
read(2) into the 40K buffer in cg_find_root() returns 4035 bytes, and the
cgroup2 line starts at offset 4953.  cg_find_root() then fails, and every
test that calls cg_find_unified_root() -- test_core, test_cpu, test_cpuset,
test_freezer, test_hugetlb_memcg, test_kill, test_kmem, test_memcontrol,
test_pids and test_zswap -- exits with SKIP "cgroup v2 isn't mounted"
without running a single test.

proc_mount_contains() searches only that same prefix, so a mount option
listed later in /proc/mounts is reported as absent, which is what the
probes in test_memcontrol and test_hugetlb_memcg are based on.

Read until the buffer is full or EOF instead.  A partial read is still
possible when max_len is too small, so callers that deliberately read a
prefix, such as cg_read_strcmp(), keep working.

Signed-off-by: Shaojie Sun <sunshaojie@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions