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From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Florian Weimer <fw@deneb.enyo.de>, David.Laight@ACULAB.COM,
        carlos@redhat.com, Peter Oskolkov <posk@posk.io>,
        libc-coord@lists.openwall.com,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: [RFC PATCH 1/3] rseq: Introduce feature size and alignment ELF auxiliary vector entries
Date:   Thu,  3 Feb 2022 14:38:51 -0500
Message-Id: <20220203193853.21511-1-mathieu.desnoyers@efficios.com>
Precedence: bulk

Export the rseq feature size supported by the kernel as well as the
required allocation alignment for the rseq per-thread area to user-space
through ELF auxiliary vector entries.

This is part of the extensible rseq ABI.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 fs/binfmt_elf.c             | 5 +++++
 include/uapi/linux/auxvec.h | 2 ++
 include/uapi/linux/rseq.h   | 5 +++++
 3 files changed, 12 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 605017eb9349..77776582e76d 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -46,6 +46,7 @@
 #include <linux/cred.h>
 #include <linux/dax.h>
 #include <linux/uaccess.h>
+#include <linux/rseq.h>
 #include <asm/param.h>
 #include <asm/page.h>
 
@@ -286,6 +287,10 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	if (bprm->have_execfd) {
 		NEW_AUX_ENT(AT_EXECFD, bprm->execfd);
 	}
+#ifdef CONFIG_RSEQ
+	NEW_AUX_ENT(AT_RSEQ_FEATURE_SIZE, offsetof(struct rseq, end));
+	NEW_AUX_ENT(AT_RSEQ_ALIGN, __alignof__(struct rseq));
+#endif
 #undef NEW_AUX_ENT
 	/* AT_NULL is zero; clear the rest too */
 	memset(elf_info, 0, (char *)mm->saved_auxv +
diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index c7e502bf5a6f..6991c4b8ab18 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h
@@ -30,6 +30,8 @@
 				 * differ from AT_PLATFORM. */
 #define AT_RANDOM 25	/* address of 16 random bytes */
 #define AT_HWCAP2 26	/* extension of AT_HWCAP */
+#define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
+#define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
 
 #define AT_EXECFN  31	/* filename of program */
 
diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 77ee207623a9..05d3c4cdeb40 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -130,6 +130,11 @@ struct rseq {
 	 *     this thread.
 	 */
 	__u32 flags;
+
+	/*
+	 * Flexible array member at end of structure, after last feature field.
+	 */
+	char end[];
 } __attribute__((aligned(4 * sizeof(__u64))));
 
 #endif /* _UAPI_LINUX_RSEQ_H */
-- 
2.17.1