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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-9141e27b5c3sm3228816d6.30.2026.09.23.14.27.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Sep 2026 14:27:23 -0700 (PDT) Date: Wed, 23 Sep 2026 17:27:21 -0400 From: Gregory Price To: Park Tae-sun Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , Brendan Jackman , Muhammad Usama Anjum , linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] selftests/mm: fix mlock2 errno handling and false PASS on ENOSYS Message-ID: References: <20260923-selftests-mm-mlock2-fix-v1-1-750b627854c6@dgu.ac.kr> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260923-selftests-mm-mlock2-fix-v1-1-750b627854c6@dgu.ac.kr> X-Rspam-User: X-Stat-Signature: qxs5mq97gk5tfpnazia7nf655digf3hp X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: B2C6940009 X-HE-Tag: 1790198845-199199 X-HE-Meta: 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 3OKSAo7w osp5wtBjtPN4WehftNpv+KJEzL23Fe44hbwIxGvuU42qLCE3p8R8veocwSCm8tDnJvVf259uIl4Jv76rQ2BiLVIdH3q2VzBoLlwKozhk0d8Sokl2f6ZgygWGswNZSr8p5quGxd1T9w4CxdQAWZebOHRkKoGYIqkmEO35LxiWIn7Qxz+KcHnmsrNQ6cOBCdQ0+LRCTIyAh6PasfpnfNq0kh7Y+2bT2vCOZfSFBZVSJRvLS9xM/QARkhOgZWLnYQcQJrKt1KS+OIlXn6emMvG7C1lXlibMhZQ8ypds3Td5vwOzrW/EEutG22dz9fZsoHs38A8wMQDpwyJFbonO1/dWRga53W+5rOGePxN2SWgeP6HM8vowDItob+2dccYdynr6h4Q24 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 23, 2026 at 06:26:48PM +0900, Park Tae-sun wrote: > While inspecting selftests/mm syscall wrappers, I noticed that mlock2_() > in mlock2.h handles the syscall return value differently from other > wrappers: > > int ret = syscall(__NR_mlock2, start, len, flags); > if (ret) { > errno = ret; > return -1; > } > > Commit 1ddae9d67ee1 ("selftests/mm/mlock: print error on failure") > introduced this intending to make mlock2_() behave like libc by setting > errno and returning -1. However, glibc syscall(2) already returns -1 on > failure and sets positive errno. Assigning "errno = ret;" overwrites > errno with -1. > > To verify this, mlock2 was disabled in the kernel (via sys_ni_syscall) to > return -ENOSYS. Testing revealed two interrelated defects: > > 1. In the unmodified test, mlock2_() clobbered errno to -1. The check > "if (ret && errno == ENOSYS)" in main() was bypassed, resulting in an > immediate crash in the first test: > > ~ # ./mlock2-tests > TAP version 13 > 1..15 > Bail out! mlock2(0): Unknown error -1 > # Planned tests != run tests (15 != 0) > # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0 > (exit code: 1 - FAIL) > > 2. After restoring mlock2_() to directly return syscall(), errno correctly > retained ENOSYS (38), entering the ENOSYS check in main(). However, it > then called ksft_finished(): > > ~ # ./mlock2-tests > TAP version 13 > # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0 > ~ # echo $? > 0 > > Because ksft_set_plan() had not been called yet (ksft_plan == 0) and > zero tests ran (ksft_pass == 0), ksft_finished() evaluated 0 == 0 as > success and exited with KSFT_PASS (code 0) without any TAP skip header. > > Fix both issues by: > 1. Returning the syscall() result directly in mlock2_() so that errno is > preserved. > 2. Calling ksft_exit_skip() on ENOSYS so unsupported kernels report a TAP > skip ("1..0 # SKIP ...") and exit with KSFT_SKIP (code 4). > > Verification on the mlock2-disabled kernel: > > ~ # ./mlock2-tests > TAP version 13 > 1..0 # SKIP mlock2() syscall is not supported > ~ # echo $? > 4 > > Re-enabling mlock2 in the kernel confirmed all 15 tests pass cleanly: > > ~ # ./mlock2-tests > TAP version 13 > 1..15 > ok 1 test_mlock_lock: Locked > ... > ok 15 test_mlockall_future_droppable: droppable memory not locked > # Totals: pass:15 fail:0 xfail:0 xpass:0 skip:0 error:0 > ~ # echo $? > 0 > > Fixes: 1ddae9d67ee1 ("selftests/mm/mlock: print error on failure") > Fixes: 65c89684896d ("selftests/mm: mlock2-tests: conform test to TAP format output") > Signed-off-by: Park Tae-sun Reviewed-by: Gregory Price