From ae9cafd919be2f5b1b44ca7afe66e1656e0acdb6 Mon Sep 17 00:00:00 2001
From: Conrad Meyer <cem@FreeBSD.org>
Date: Fri, 30 Oct 2020 19:02:59 +0000
Subject: [PATCH] linux(4): Quiesce warning about madvise(..., -1)

This API misuse is intended to produce an error value to detect certain
bogus stub implementations of MADV_WIPEONFORK.  We don't need to log a
warning about it.

Example:
https://boringssl.googlesource.com/boringssl/+/ad5582985cc6b89d0e7caf0d9cc7e301de61cf66%5E%21/

Reviewed by:	emaste, trasz
Differential Revision:	https://reviews.freebsd.org/D27017
---
 sys/compat/linux/linux_mmap.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/sys/compat/linux/linux_mmap.c b/sys/compat/linux/linux_mmap.c
index cc63c45579a..1b4b0b78280 100644
--- a/sys/compat/linux/linux_mmap.c
+++ b/sys/compat/linux/linux_mmap.c
@@ -394,6 +394,16 @@ linux_madvise_common(struct thread *td, uintptr_t addr, size_t len, int behav)
 	case LINUX_MADV_SOFT_OFFLINE:
 		linux_msg(curthread, "unsupported madvise MADV_SOFT_OFFLINE");
 		return (EINVAL);
+	case -1:
+		/*
+		 * -1 is sometimes used as a dummy value to detect simplistic
+		 * madvise(2) stub implementations.  This safeguard is used by
+		 * BoringSSL, for example, before assuming MADV_WIPEONFORK is
+		 * safe to use.  Don't produce an "unsupported" error message
+		 * for this special dummy value, which is unlikely to be used
+		 * by any new advisory behavior feature.
+		 */
+		return (EINVAL);
 	default:
 		linux_msg(curthread, "unsupported madvise behav %d", behav);
 		return (EINVAL);