[Buildroot] [PATCH v2 07/12] package/busybox: fix applets runtime issue when building with clang cross-compiler

[Romain Naour <romain.naour@smile.fr>]

Apply a patch contributed by Luis Marques on the Busybox mailing list [1]
fixing a runtime issue (segfault) when busybox is compiled by Clang.

The patch disable the compiler optimizations for Clang/LLVM only.

Without this patch, busybox segfault with several applets
(login on aarch64 using Clang 8.0.1, init on x86_64 using Clang 9.0.0rc3)

[1] http://lists.busybox.net/pipermail/busybox/2019-June/087337.html

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Luis Marques <luismarques@lowrisc.org>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Valentin Korenblit <valentinkorenblit@gmail.com>
---
Note:
 The Luis Marques's SoB line in the Busybox patch is missing.
 The patch is still under review to avoid disabling optimizations.
---
 ...use-BB_GLOBAL_CONST-where-applicable.patch | 173 ++++++++++++++++++
 1 file changed, 173 insertions(+)
 create mode 100644 package/busybox/0003-use-BB_GLOBAL_CONST-where-applicable.patch

diff --git a/package/busybox/0003-use-BB_GLOBAL_CONST-where-applicable.patch b/package/busybox/0003-use-BB_GLOBAL_CONST-where-applicable.patch
new file mode 100644
index 0000000000..738a15b0de
--- /dev/null
+++ b/package/busybox/0003-use-BB_GLOBAL_CONST-where-applicable.patch
@@ -0,0 +1,173 @@
+From 6f53fa8303edff685aee3cc16a6c8967fae869db Mon Sep 17 00:00:00 2001
+From: Luis Marques <luismarques@lowrisc.org>
+Date: Wed, 4 Sep 2019 17:48:39 +0200
+Subject: [PATCH] use BB_GLOBAL_CONST where applicable
+
+Signed-off-by: Romain Naour <romain.naour@smile.fr>
+---
+ coreutils/test.c          |  3 +--
+ coreutils/test_ptr_hack.c |  2 +-
+ include/libbb.h           | 27 +++++++++++++++++++++++++--
+ libbb/lineedit.c          |  3 +--
+ libbb/lineedit_ptr_hack.c |  2 +-
+ libbb/ptr_to_globals.c    |  2 +-
+ shell/ash.c               | 13 -------------
+ shell/ash_ptr_hack.c      |  6 +++---
+ 8 files changed, 33 insertions(+), 25 deletions(-)
+
+diff --git a/coreutils/test.c b/coreutils/test.c
+index 8d7dac025..e1d440106 100644
+--- a/coreutils/test.c
++++ b/coreutils/test.c
+@@ -400,8 +400,7 @@ struct test_statics {
+ 	jmp_buf leaving;
+ };
+ 
+-/* See test_ptr_hack.c */
+-extern struct test_statics *const test_ptr_to_statics;
++extern struct test_statics *BB_GLOBAL_CONST test_ptr_to_statics;
+ 
+ #define S (*test_ptr_to_statics)
+ #define args            (S.args         )
+diff --git a/coreutils/test_ptr_hack.c b/coreutils/test_ptr_hack.c
+index 5ba9dcc68..6759b2144 100644
+--- a/coreutils/test_ptr_hack.c
++++ b/coreutils/test_ptr_hack.c
+@@ -18,6 +18,6 @@ struct test_statics *test_ptr_to_statics;
+ /* gcc -combine will see through and complain */
+ /* Using alternative method which is more likely to break
+  * on weird architectures, compilers, linkers and so on */
+-struct test_statics *const test_ptr_to_statics __attribute__ ((section (".data")));
++struct test_statics *BB_GLOBAL_CONST test_ptr_to_statics __attribute__ ((section (".data")));
+ 
+ #endif
+diff --git a/include/libbb.h b/include/libbb.h
+index 021100db1..2523fd89c 100644
+--- a/include/libbb.h
++++ b/include/libbb.h
+@@ -338,10 +338,33 @@ struct BUG_off_t_size_is_misdetected {
+ #endif
+ #endif
+ 
++/* We use a trick to have more optimized code (fewer pointer reloads). E.g.:
++ *  ash.c:   extern struct globals *const ash_ptr_to_globals;
++ *  ash_ptr_hack.c: struct globals *ash_ptr_to_globals;
++ * This way, compiler in ash.c knows the pointer cannot change.
++ *
++ * However, this relies on C undefined behavior, so we whitelist compilers
++ * where we know this isn't problematic, by using the the BB_GLOBAL_CONST
++ * preprocessor definition.
++ * If you are sure this trick also works with your toolchain you can add
++ * "-DBB_GLOBAL_CONST='const'" to CONFIG_EXTRA_CFLAGS or add your compiler to
++ * the whitelist below.
++ */
++
++#ifndef BB_GLOBAL_CONST
++# if defined(__clang__)
++#  define BB_GLOBAL_CONST
++# elif defined(__GNUC__)
++#  define BB_GLOBAL_CONST const
++# else
++#  define BB_GLOBAL_CONST
++# endif
++#endif
++
+ #if defined(__GLIBC__)
+ /* glibc uses __errno_location() to get a ptr to errno */
+ /* We can just memorize it once - no multithreading in busybox :) */
+-extern int *const bb_errno;
++extern int *BB_GLOBAL_CONST bb_errno;
+ #undef errno
+ #define errno (*bb_errno)
+ #endif
+@@ -2109,7 +2132,7 @@ struct globals;
+ /* '*const' ptr makes gcc optimize code much better.
+  * Magic prevents ptr_to_globals from going into rodata.
+  * If you want to assign a value, use SET_PTR_TO_GLOBALS(x) */
+-extern struct globals *const ptr_to_globals;
++extern struct globals *BB_GLOBAL_CONST ptr_to_globals;
+ /* At least gcc 3.4.6 on mipsel system needs optimization barrier */
+ #define barrier() __asm__ __volatile__("":::"memory")
+ #define SET_PTR_TO_GLOBALS(x) do { \
+diff --git a/libbb/lineedit.c b/libbb/lineedit.c
+index fbabc6c12..b9e9719c5 100644
+--- a/libbb/lineedit.c
++++ b/libbb/lineedit.c
+@@ -180,8 +180,7 @@ struct lineedit_statics {
+ #endif
+ };
+ 
+-/* See lineedit_ptr_hack.c */
+-extern struct lineedit_statics *const lineedit_ptr_to_statics;
++extern struct lineedit_statics *BB_GLOBAL_CONST lineedit_ptr_to_statics;
+ 
+ #define S (*lineedit_ptr_to_statics)
+ #define state            (S.state           )
+diff --git a/libbb/lineedit_ptr_hack.c b/libbb/lineedit_ptr_hack.c
+index dc45855d5..ac33bd409 100644
+--- a/libbb/lineedit_ptr_hack.c
++++ b/libbb/lineedit_ptr_hack.c
+@@ -18,6 +18,6 @@ struct lineedit_statics *lineedit_ptr_to_statics;
+ /* gcc -combine will see through and complain */
+ /* Using alternative method which is more likely to break
+  * on weird architectures, compilers, linkers and so on */
+-struct lineedit_statics *const lineedit_ptr_to_statics __attribute__ ((section (".data")));
++struct lineedit_statics *BB_GLOBAL_CONST lineedit_ptr_to_statics __attribute__ ((section (".data")));
+ 
+ #endif
+diff --git a/libbb/ptr_to_globals.c b/libbb/ptr_to_globals.c
+index 8ba9cd154..26d7b2042 100644
+--- a/libbb/ptr_to_globals.c
++++ b/libbb/ptr_to_globals.c
+@@ -25,7 +25,7 @@ int *bb_errno;
+ /* gcc -combine will see through and complain */
+ /* Using alternative method which is more likely to break
+  * on weird architectures, compilers, linkers and so on */
+-struct globals *const ptr_to_globals __attribute__ ((section (".data")));
++struct globals *BB_GLOBAL_CONST ptr_to_globals __attribute__ ((section (".data")));
+ 
+ #ifdef __GLIBC__
+ int *const bb_errno __attribute__ ((section (".data")));
+diff --git a/shell/ash.c b/shell/ash.c
+index e3bbac9a0..3141f3812 100644
+--- a/shell/ash.c
++++ b/shell/ash.c
+@@ -288,19 +288,6 @@ typedef long arith_t;
+ # error "Do not even bother, ash will not run on NOMMU machine"
+ #endif
+ 
+-/* We use a trick to have more optimized code (fewer pointer reloads):
+- *  ash.c:   extern struct globals *const ash_ptr_to_globals;
+- *  ash_ptr_hack.c: struct globals *ash_ptr_to_globals;
+- * This way, compiler in ash.c knows the pointer can not change.
+- *
+- * However, this may break on weird arches or toolchains. In this case,
+- * set "-DBB_GLOBAL_CONST=''" in CONFIG_EXTRA_CFLAGS to disable
+- * this optimization.
+- */
+-#ifndef BB_GLOBAL_CONST
+-# define BB_GLOBAL_CONST const
+-#endif
+-
+ 
+ /* ============ Hash table sizes. Configurable. */
+ 
+diff --git a/shell/ash_ptr_hack.c b/shell/ash_ptr_hack.c
+index f69840825..af16cca27 100644
+--- a/shell/ash_ptr_hack.c
++++ b/shell/ash_ptr_hack.c
+@@ -22,8 +22,8 @@ struct globals_var      *ash_ptr_to_globals_var;
+ /* gcc -combine will see through and complain */
+ /* Using alternative method which is more likely to break
+  * on weird architectures, compilers, linkers and so on */
+-struct globals_misc     *const ash_ptr_to_globals_misc __attribute__ ((section (".data")));
+-struct globals_memstack *const ash_ptr_to_globals_memstack __attribute__ ((section (".data")));
+-struct globals_var      *const ash_ptr_to_globals_var __attribute__ ((section (".data")));
++struct globals_misc     *BB_GLOBAL_CONST ash_ptr_to_globals_misc __attribute__ ((section (".data")));
++struct globals_memstack *BB_GLOBAL_CONST ash_ptr_to_globals_memstack __attribute__ ((section (".data")));
++struct globals_var      *BB_GLOBAL_CONST ash_ptr_to_globals_var __attribute__ ((section (".data")));
+ 
+ #endif
+-- 
+2.21.0
+
-- 
2.21.0