linux-hardening - [PATCH 4/4] aarch64: Remove the bti link

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Date:   Tue,  3 Nov 2020 10:26:42 +0000
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     libc-alpha@...rceware.org
Cc:     Jeremy Linton <jeremy.linton@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Brown <broonie@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org
Subject: [PATCH 4/4] aarch64: Remove the bti link_map field [BZ #26831]

The bti link_map field is no longer necessary because PROT_BTI
is applied at note processing time immediately instead of in
_dl_open_check based on the bti field.

This is a separate patch that is not expected to be backported
to avoid changing the link_map layout that is libc internal ABI.
---
 sysdeps/aarch64/dl-prop.h | 5 +----
 sysdeps/aarch64/linkmap.h | 1 -
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/sysdeps/aarch64/dl-prop.h b/sysdeps/aarch64/dl-prop.h
index 762bc93733..cf14381e4a 100644
--- a/sysdeps/aarch64/dl-prop.h
+++ b/sysdeps/aarch64/dl-prop.h
@@ -52,10 +52,7 @@ _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
 
       unsigned int feature_1 = *(unsigned int *) data;
       if (feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)
-	{
-	  l->l_mach.bti = true;  /* No longer needed.  */
-	  _dl_bti_protect (l, fd);
-	}
+	_dl_bti_protect (l, fd);
 
       /* Stop if we processed the property note.  */
       return 0;
diff --git a/sysdeps/aarch64/linkmap.h b/sysdeps/aarch64/linkmap.h
index 847a03ace2..e921e77495 100644
--- a/sysdeps/aarch64/linkmap.h
+++ b/sysdeps/aarch64/linkmap.h
@@ -22,5 +22,4 @@ struct link_map_machine
 {
   ElfW(Addr) plt;	  /* Address of .plt */
   void *tlsdesc_table;	  /* Address of TLS descriptor hash table.  */
-  bool bti;		  /* Branch Target Identification is enabled.  */
 };
-- 
2.17.1