3 Jun
2021
3 Jun
'21
5:40 a.m.
This patch fixes segfault of all user space processes (including init, which caused a
panic) on recent buildroot powerpc32 builds.
The issue has been reported by Romain Naour in this thread:
https://mailman.uclibc-ng.org/pipermail/devel/2021-May/002068.html
Recent buildroot toolchain enables secure PLT in powerpc gcc.
The latter will then supply -msecure-plt to gas invocations by default.
Recent buildroot also enables PIE by defaults.
For the secure PLT to work in PIC, the r30 register needs to point to the GOT.
Old "bss plt" was just a one-instruction-wide PLT slot, pointed-to by a
R_PPC_JMP_SLOT relocation, which was written on-the-fly to contain a branch instruction to
the correct address. It therefore had to stay writable.
New secure PLT only contains read-only code which loads the branch address from the
writable GOT.
Note: secure PLT without PIC does not need r30 to be set. Because offset between plt stub
code and got is known at link-time. In this case the PLT entry looks like:
1009b3e0 <__uClibc_main@plt>:
1009b3e0: 3d 60 10 0e lis r11,4110
1009b3e4: 81 6b 03 74 lwz r11,884(r11)
1009b3e8: 7d 69 03 a6 mtctr r11
1009b3ec: 4e 80 04 20 bctr
Whereas secure PLT with PIC - offset between plt and got is unknown at link-time - looks
like this:
000af800 <00000000.plt_pic32.__uClibc_main>:
af800: 81 7e 03 80 lwz r11,896(r30)
af804: 7d 69 03 a6 mtctr r11
af808: 4e 80 04 20 bctr
af80c: 60 00 00 00 nop
Signed-off-by: Yann Sionneau <yann(a)sionneau.net>
---
Rules.mak | 3 ++-
ldso/ldso/powerpc/dl-startup.h | 4 ++++
libc/sysdeps/linux/powerpc/crt1.S | 4 ++++
3 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/Rules.mak b/Rules.mak
index 1fa09be23..c789aefd3 100644
--- a/Rules.mak
+++ b/Rules.mak
@@ -483,9 +483,10 @@ ifeq ($(TARGET_ARCH),powerpc)
PICFLAG:=-fpic
PIEFLAG_NAME:=-fpie
PPC_HAS_REL16:=$(shell printf "\t.text\n\taddis
11,30,_GLOBAL_OFFSET_TABLE_-.@ha\n" | $(CC) -c -x assembler -o /dev/null - 2>
/dev/null && echo -n y || echo -n n)
+ PPC_HAS_SECUREPLT:=$(shell $(CC) --verbose 2>&1 | grep -- --enable-secureplt >
/dev/null && echo -n y || echo -n n)
+ CPU_CFLAGS-$(PPC_HAS_SECUREPLT) += -DPPC_HAS_SECUREPLT
CPU_CFLAGS-$(PPC_HAS_REL16)+= -DHAVE_ASM_PPC_REL16
CPU_CFLAGS-$(CONFIG_E500) += "-D__NO_MATH_INLINES"
-
endif
ifeq ($(TARGET_ARCH),bfin)
diff --git a/ldso/ldso/powerpc/dl-startup.h b/ldso/ldso/powerpc/dl-startup.h
index 8b2a517e2..8503350c5 100644
--- a/ldso/ldso/powerpc/dl-startup.h
+++ b/ldso/ldso/powerpc/dl-startup.h
@@ -25,6 +25,10 @@ __asm__(
#else
" bl _GLOBAL_OFFSET_TABLE_-4@local\n" /* Put our GOT pointer in r31, */
" mflr 31\n"
+#endif
+/* I'm quite sure this piece of code is always compiled as PIC but let's be sure
*/
+#if defined(PPC_HAS_SECUREPLT) && defined(__PIC__)
+ " mr 30,31\n"
#endif
" addi 1,1,16\n" /* Restore SP */
" lwz 7,_dl_skip_args@got(31)\n" /* load EA of _dl_skip_args */
diff --git a/libc/sysdeps/linux/powerpc/crt1.S b/libc/sysdeps/linux/powerpc/crt1.S
index 27bfc5a5a..3f5d056c0 100644
--- a/libc/sysdeps/linux/powerpc/crt1.S
+++ b/libc/sysdeps/linux/powerpc/crt1.S
@@ -56,6 +56,10 @@ _start:
# else
bl _GLOBAL_OFFSET_TABLE_-4@local
mflr r31
+# endif
+ /* in PIC/PIE, plt stubs need r30 to point to the GOT if using secure-plt */
+# ifdef PPC_HAS_SECUREPLT
+ mr 30,31
# endif
#endif
/* Set up the small data pointer in r13. */
--
2.25.1