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[68.48.65.54]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8a593908b47sm186312926d6.11.2026.04.07.11.15.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Apr 2026 11:15:35 -0700 (PDT) From: Michael Bommarito To: Richard Weinberger , Anton Ivanov , Johannes Berg Cc: linux-um@lists.infradead.org, linux-kernel@vger.kernel.org, Michael Bommarito , stable@vger.kernel.org Subject: [PATCH v2] um: drivers: call kernel_strrchr() explicitly in cow_user.c Date: Tue, 7 Apr 2026 14:15:28 -0400 Message-ID: <20260407181528.879358-1-michael.bommarito@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260407164435.726012-1-michael.bommarito@gmail.com> References: <20260407164435.726012-1-michael.bommarito@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Building ARCH=um on a host with glibc >= 2.43 fails: arch/um/drivers/cow_user.c:156:17: error: implicit declaration of function 'strrchr' [-Wimplicit-function-declaration] cow_user.o is a host-side helper (compiled with -D__UM_HOST__) that calls strrchr(). It inherits the global -Dstrrchr=kernel_strrchr remap from arch/um/Makefile, which is intentionally kept in USER_CFLAGS to prevent libc/kernel symbol clashes. This combination was harmless until glibc 2.43, which added (glibc commit cd748a63ab1a, "Implement C23 const-preserving standard library macros"): #define strrchr(S,C) __glibc_const_generic(S, const char *, strrchr(S, C)) The glibc function-like macro replaces the -D object-like macro. The inner strrchr token in the expansion is protected from recursive expansion, so it refers to the bare symbol strrchr -- but the header declaration was already rewritten to kernel_strrchr by the -D. The result is an implicit-declaration error. The global -Dstrrchr=kernel_strrchr remap was originally added in commit 2c51a4bc0233 ("um: fix strrchr() problems") to resolve a linker clash when both CONFIG_STATIC_LINK and CONFIG_UML_NET_VDE are set. Recently, commit a74b6c0e53a6 ("um: Don't rename vmap to kernel_vmap") trimmed the now-obsolete vmap remap and updated the comment in arch/um/Makefile to explicitly call out -Dstrrchr=kernel_strrchr as one of the remaps that still prevents libc symbol clashes. That global remap stays in place. Rather than exempting cow_user.o from the remap at build time, call kernel_strrchr() explicitly in the source. This is slightly more honest about which strrchr the code wants (the kernel's, as it has been since 2011), sidesteps the interaction with glibc's C23 macro entirely, avoids adding a new libc strrchr dependency to the UML binary, and is robust to future C23 const-preserving macros for strchr, memchr, strstr, etc. cow_user.o is built whenever CONFIG_BLK_DEV_UBD=y (the standard UML block device), so this affects most non-trivial UML configurations. cow_user.c is the only file under arch/um/ that calls strrchr(), so no other translation units need changes. Standalone reproducer (fails on glibc >= 2.43, succeeds on older): printf '#include \nvoid f(void) { char *p = strrchr("foo", 47); }\n' \ | gcc -c -Dstrrchr=kernel_strrchr -x c - -o /dev/null Tested on: - Host: Ubuntu, glibc 2.43-2ubuntu1, gcc 15.2.0 - Kernel: v7.0-rc6 (3aae9383f42f); verified that neither arch/um/drivers/Makefile nor arch/um/drivers/cow_user.c changed between rc6 and rc7, so the fix applies and behaves identically on both - Build: ARCH=um defconfig + CONFIG_BLK_DEV_UBD=y, clean compile with no warnings - nm: cow_user.o references 'U kernel_strrchr' (not libc strrchr), and the final linux binary has no strrchr@GLIBC_2.2.5 symbol anywhere; kernel_strrchr is defined exactly once by lib/string.o and EXPORT_SYMBOL'd - Boot: UML boots to Debian bookworm multi-user and graphical targets with a COW overlay (ubd0=cow,backing), which exercises the patched absolutize() -> kernel_strrchr() code path in cow_user.c AI coding tools (Claude Code with Opus 4.6, and Codex with GPT-5.4) assisted with debugging, test design, and drafting; the author manually reviewed every line and executed every build and boot test on the host. Full disclosure was posted with v1; a shorter summary is in the Assisted-by: trailers below. Fixes: 2c51a4bc0233 ("um: fix strrchr() problems") Suggested-by: Johannes Berg Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-6 Assisted-by: Codex:gpt-5-4 Signed-off-by: Michael Bommarito --- v1: https://lore.kernel.org/all/20260407164435.726012-2-michael.bommarito@gmail.com/ Review: https://lore.kernel.org/linux-um/1e15d25c23b444eae1dcfc01432e7ec1e19e25a0.camel@sipsolutions.net/ Changes since v1: - Per Johannes Berg's review (link above): rather than exempting cow_user.o from the global -Dstrrchr=kernel_strrchr remap via -Ustrrchr in arch/um/drivers/Makefile, call kernel_strrchr() explicitly in cow_user.c. This keeps the existing semantic that cow_user.o uses the kernel's strrchr (no new libc dependency on the host side), and the source no longer relies on the build-time rewrite at all. - Reverted the arch/um/drivers/Makefile CFLAGS change from v1. - Verified locally on v7.0-rc6: clean build, cow_user.o references 'U kernel_strrchr' (no libc strrchr), the final linux binary has no strrchr@GLIBC_2.2.5 reference anywhere, and the kernel boots to multi-user with a COW overlay that exercises the patched code path. Full boot log captured locally. arch/um/drivers/cow_user.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/um/drivers/cow_user.c b/arch/um/drivers/cow_user.c index 29b46581ddd1..ec8e6121b402 100644 --- a/arch/um/drivers/cow_user.c +++ b/arch/um/drivers/cow_user.c @@ -15,6 +15,12 @@ #include "cow.h" #include "cow_sys.h" +/* + * arch/um/Makefile remaps strrchr to kernel_strrchr; call the kernel + * name directly to avoid glibc >= 2.43's C23 strrchr macro. + */ +extern char *kernel_strrchr(const char *, int); + #define PATH_LEN_V1 256 /* unsigned time_t works until year 2106 */ @@ -153,7 +159,7 @@ static int absolutize(char *to, int size, char *from) errno); return -1; } - slash = strrchr(from, '/'); + slash = kernel_strrchr(from, '/'); if (slash != NULL) { *slash = '\0'; if (chdir(from)) { -- 2.49.0