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b=Y678PVTNPmYtPnycSwJKcSUOIWLY8g0nCVmTZwaQWKaxPahhGZgP8sdmNu9WcCYcUJ 9/1KrA1fOGwBYKa7a9zGYTEQwhTguaufpsLKVeTq02COY3L4XLaESk2nRm4GuXR2tp0B 3t7rL/4qXBci/oh+CgEpO/0cVmnEXM75r8ov/yEI0Dk6SM01c/m0Y2BbzU2v35JzMk/U y9EDQyIy9OnnyYu9mlv8LSUYtvo+Ite8kAThNXxt4dyMjV+lrPsN9Q2jhbS85jDg7PCG ZCMcuR0CQmjujtkN4lJESB0N33M9TlDtcZv4rVn/K0UQgQ0beD3VgTmjW1LKSuVSh7Sy 3mzQ== X-Forwarded-Encrypted: i=1; AKwUvBzYFUZqBLxGZ22ycg/fhi8xwTRLW3DDtb9B1BxU+ICC+M5Qzx+uYJ1l9Z6t3N8bV6Hp7CqpN4+7RqVzsw==@vger.kernel.org X-Gm-Message-State: AFq9FYJMXHOC27rxTXBtT7z9U4didYpQMByhD7CJOge+oSUnL54wYyV8 Cr58ur4z+phU8QwfiDtKfVkhHjulrVP98mTvaQVkzYQvnsOAu1OorfAM6D63GxOncv45VAkj4rQ DadnsDNYwbFIj/w3x7rB3+w== X-Received: from pjbst6.prod.google.com ([2002:a17:90b:1fc6:b0:3ac:989f:b65f]) (user=jthoughton job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:1a92:b0:3a8:211b:6c8c with SMTP id 98e67ed59e1d1-3adeaae6e15mr7057279a91.22.1793051576336; Mon, 26 Oct 2026 14:52:56 -0700 (PDT) Date: Mon, 26 Oct 2026 21:52:26 +0000 In-Reply-To: <20261026215226.3713941-1-jthoughton@google.com> X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 References: <20261026215226.3713941-1-jthoughton@google.com> X-Mailer: git-send-email 2.56.0.307.g5fe7bd680c-goog Message-ID: <20261026215226.3713941-11-jthoughton@google.com> Subject: [RFC PATCH 10/10] uaccess: Move the NULL mm warning out of line From: James Houghton To: Andrew Morton , Alexander Viro Cc: Thomas Gleixner , Peter Zijlstra , Kees Cook , Arnd Bergmann , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Sean Christopherson , Paolo Bonzini , Jim Mattson , Jens Axboe , Pavel Begunkov , "James E.J. Bottomley" , "Martin K. Petersen" , David Sterba , Chris Mason , Alexei Starovoitov , Daniel Borkmann , John Fastabend , Josh Poimboeuf , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, io-uring@vger.kernel.org, linux-scsi@vger.kernel.org, linux-btrfs@vger.kernel.org, bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, James Houghton Content-Type: text/plain; charset="UTF-8" uaccess_without_mm() is inlined into every user copy, including every copy_{to,from}_user() call site on architectures that define INLINE_COPY_USER, such as arm64. Each inlined WARN_ON_ONCE() adds its own __bug_table entry. Keep the NULL current->mm check inline, but move the warning into a single out-of-line, __cold function, uaccess_warn_without_mm(). The trade-off is that only the first offending access in the system warns, rather than the first one from each call site. The stack trace still identifies the caller, and the access is refused either way. Compared to the base of this series, with defconfig + KVM: inline WARN_ON_ONCE() out-of-line warning x86_64 total +5,884 B (+0.014%) +9,714 B (+0.023%) arm64 total +40,948 B (+0.10%) +24,609 B (+0.06%) On arm64, this removes the ~16 KB of __bug_table growth, while text size is unchanged, as a call is the same size as the inline brk. On x86_64, the call is larger than the inline ud2 and causes more .cold splitting, so this is a slight regression. Signed-off-by: James Houghton --- NOTE: This patch is included for reference only, to show the cost of the inline warning. --- include/linux/uaccess.h | 8 +++++++- lib/usercopy.c | 6 ++++++ 2 files changed, 13 insertions(+), 1 deletion(-) diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h index 7d26d2de9599..6a5b6c2b3388 100644 --- a/include/linux/uaccess.h +++ b/include/linux/uaccess.h @@ -66,9 +66,15 @@ * that keeps user addresses belonging to a specific mm must check * current->mm against that mm itself (see e.g. KVM's kvm->mm checks). */ +void __cold uaccess_warn_without_mm(void); + static __always_inline bool uaccess_without_mm(void) { - return WARN_ON_ONCE(!current->mm); + if (likely(current->mm)) + return false; + /* Warn out of line to keep inlined user copies small. */ + uaccess_warn_without_mm(); + return true; } /* diff --git a/lib/usercopy.c b/lib/usercopy.c index 9146c729c47a..3d547ecb74df 100644 --- a/lib/usercopy.c +++ b/lib/usercopy.c @@ -26,6 +26,12 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n) EXPORT_SYMBOL(_copy_to_user); #endif +void __cold uaccess_warn_without_mm(void) +{ + WARN_ON_ONCE(1); +} +EXPORT_SYMBOL(uaccess_warn_without_mm); + /** * check_zeroed_user: check if a userspace buffer only contains zero bytes * @from: Source address, in userspace. -- 2.56.0.307.g5fe7bd680c-goog