kernel: fix objtool build on macOS hosts

Since kernel 5a406031d0719 (2025-03-17, in 6.15), objtool calls
sendfile, but sendfile is Linux-specific and a sendfile with the
requisite capabilities is not available on other operating systems.
Provide a more portable fallback implementation to be used when
cross-building on non-Linux hosts.

This fixes:

  CC      …/linux-6.18.33/tools/objtool/builtin-check.o
builtin-check.c:13:10: fatal error: 'sys/sendfile.h' file
      not found
   13 | #include <sys/sendfile.h>
      |          ^~~~~~~~~~~~~~~~
1 error generated.
gmake[8]: *** […/linux-6.18.33/tools/build/Makefile.build:86: …/linux-6.18.33/tools/objtool/builtin-check.o] Error 1

Fixes: https://github.com/openwrt/openwrt/issues/23356
Signed-off-by: Mark Mentovai <mark@mentovai.com>
Link: https://github.com/openwrt/openwrt/pull/23612
Signed-off-by: Robert Marko <robimarko@gmail.com>

diff --git a/target/linux/generic/hack-6.18/202-objtool_portability.patch b/target/linux/generic/hack-6.18/202-objtool_portability.patch
new file mode 100644 (file)
index 0000000..d8ba87b
--- /dev/null
+++ b/target/linux/generic/hack-6.18/202-objtool_portability.patch
@@ -0,0 +1,91 @@
+From dc5565c4478135999d8bb193bd9dfb4f8ef5f96b Mon Sep 17 00:00:00 2001
+From: Mark Mentovai <mark@mentovai.com>
+Date: Mon, 1 Jun 2026 09:11:43 -0400
+Subject: [PATCH] objtool: fix build on macOS hosts
+
+Since 5a406031d0719 (2025-03-17, in 6.15), objtool calls sendfile, but
+sendfile is Linux-specific and a sendfile with the requisite
+capabilities is not available on other operating systems. Provide a more
+portable fallback implementation to be used when cross-building on
+non-Linux hosts.
+
+Signed-off-by: Mark Mentovai <mark@mentovai.com>
+---
+ tools/objtool/builtin-check.c | 40 +++++++++++++++++++++++++++++++++++
+ 1 file changed, 40 insertions(+)
+
+--- a/tools/objtool/builtin-check.c
++++ b/tools/objtool/builtin-check.c
+@@ -10,7 +10,9 @@
+ #include <unistd.h>
+ #include <errno.h>
+ #include <sys/stat.h>
++#ifdef __linux__
+ #include <sys/sendfile.h>
++#endif
+ #include <objtool/builtin.h>
+ #include <objtool/objtool.h>
+ #include <objtool/warn.h>
+@@ -187,12 +189,28 @@ static bool opts_valid(void)
+       return false;
+ }
+ 
++#ifndef __linux__
++#define RETRY_EINTR(x)                                                  \
++      ({                                                              \
++              __typeof__(x) eintr_wrapper_result;                     \
++              do {                                                    \
++                      eintr_wrapper_result = (x);                     \
++              } while (eintr_wrapper_result == -1 && errno == EINTR); \
++              eintr_wrapper_result;                                   \
++      })
++#endif
++
+ static int copy_file(const char *src, const char *dst)
+ {
+       size_t to_copy, copied;
+       int dst_fd, src_fd;
+       struct stat stat;
++#ifdef __linux__
+       off_t offset = 0;
++#else
++      char buf[65536];
++      ssize_t nread, nwrote;
++#endif
+ 
+       src_fd = open(src, O_RDONLY);
+       if (src_fd == -1) {
+@@ -217,11 +235,33 @@ static int copy_file(const char *src, co
+       }
+ 
+       for (to_copy = stat.st_size; to_copy > 0; to_copy -= copied) {
++#ifdef __linux__
+               copied = sendfile(dst_fd, src_fd, &offset, to_copy);
+               if (copied == -1) {
+                       ERROR_GLIBC("sendfile");
+                       return 1;
+               }
++#else
++              nread = RETRY_EINTR(
++                      read(src_fd, buf,
++                           to_copy < sizeof(buf) ? to_copy : sizeof(buf)));
++              if (nread < 0) {
++                      ERROR_GLIBC("read");
++                      return 1;
++              }
++              if (nread == 0) {
++                      ERROR("unexpected premature EOF");
++                      return 1;
++              }
++              for (copied = 0; copied < nread; copied += nwrote) {
++                      nwrote = RETRY_EINTR(
++                              write(dst_fd, buf + copied, nread - copied));
++                      if (nwrote < 0) {
++                              ERROR_GLIBC("write");
++                              return 1;
++                      }
++              }
++#endif
+       }
+ 
+       close(dst_fd);