[PATCH] um: drivers: use libc strrchr() in cow_user.o

[Michael Bommarito <michael.bommarito@gmail.com>]

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 libc strrchr().  It inherits the global -Dstrrchr=kernel_strrchr
remap from arch/um/Makefile, which is intentionally kept in USER_CFLAGS
to prevent linker clashes between libc and kernel symbols.

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 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 from arch/um/Makefile and updated the
comment to explicitly call out -Dstrrchr=kernel_strrchr as one of the
remaps that still prevents libc symbol clashes.  That framing is kept
here: the global strrchr remap is still needed for kernel-side
objects, but cow_user.o is host-side and should use libc strrchr
directly.

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().

Fix this by undoing the remap for just this translation unit via
per-object CFLAGS.  In UML's Makefile.rules, CFLAGS_$(basetarget).o
is appended after USER_CFLAGS, so -Ustrrchr correctly overrides the
earlier -Dstrrchr=kernel_strrchr.

Standalone reproducer (fails on glibc >= 2.43, succeeds on older):

  printf '#include <string.h>\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: v7.0.0-rc6 (3aae9383f42f)
  - Build: ARCH=um defconfig + CONFIG_BLK_DEV_UBD=y, clean compile
  - Boot: UML boots to Debian bookworm multi-user target
  - COW: UML boots with COW overlay (ubd0=cow,backing), exercising
    the absolutize() -> strrchr() 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 in the cover letter.

Fixes: 2c51a4bc0233 ("um: fix strrchr() problems")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 arch/um/drivers/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/um/drivers/Makefile b/arch/um/drivers/Makefile
index 36dc57840..e387ae33f 100644
--- a/arch/um/drivers/Makefile
+++ b/arch/um/drivers/Makefile
@@ -49,6 +49,9 @@ obj-$(CONFIG_UML_PCI_OVER_VFIO) += vfio_uml.o
 # pcap_user.o must be added explicitly.
 USER_OBJS := fd.o null.o pty.o tty.o xterm.o vector_user.o
 CFLAGS_null.o = -DDEV_NULL=$(DEV_NULL_PATH)
+# cow_user.o is a host-side helper that uses libc strrchr(); undo the global
+# UML remap to kernel_strrchr for this translation unit.
+CFLAGS_cow_user.o += -Ustrrchr

 CFLAGS_xterm.o += '-DCONFIG_XTERM_CHAN_DEFAULT_EMULATOR="$(CONFIG_XTERM_CHAN_DEFAULT_EMULATOR)"'

--
2.49.0