On Wed, 2023-11-08 at 12:04 +0800, WANG Rui wrote:
> When compiling module with GCC, the option `-mdirect-extern-access` is
> disabled by default. The Clang option `-fdirect-access-external-data`
> is enabled by default, so it needs to be explicitly disabled.
I'm wondering why it's enabled by default.
For this simple test case:
extern char **environ;
int main()
{
__builtin_printf("%10s\n", environ[0]);
}
With Clang 17.0.4 and "clang t1.c -S -O2", it compiles to:
main:
addi.d $sp, $sp, -16
st.d $ra, $sp, 8
pcalau12i $a0, %got_pc_hi20(environ)
ld.d $a0, $a0, %got_pc_lo12(environ)
ld.d $a0, $a0, 0
ld.d $a1, $a0, 0
pcalau12i $a0, %pc_hi20(.L.str)
addi.d $a0, $a0, %pc_lo12(.L.str)
bl %plt(printf)
move $a0, $zero
ld.d $ra, $sp, 8
addi.d $sp, $sp, 16
ret
So GOT is used for accessing the external variable environ. With "clang
t1.c -S -O2 -fdirect-access-external-data", we get:
main:
addi.d $sp, $sp, -16
st.d $ra, $sp, 8
pcalau12i $a0, %pc_hi20(environ)
addi.d $a0, $a0, %pc_lo12(environ)
ld.d $a0, $a0, 0
ld.d $a1, $a0, 0
pcalau12i $a0, %pc_hi20(.L.str)
addi.d $a0, $a0, %pc_lo12(.L.str)
bl %plt(printf)
move $a0, $zero
ld.d $ra, $sp, 8
addi.d $sp, $sp, 16
ret
then the linked binary triggers a SIGBUS. Ideally this should be
detected by the linker at link time, but currently the BFD linker fails
to detect this error (FWIW this flaw is caused by a really nasty method
for the medium code model implementation). So to me -fno-direct-access-
external-data is the default. I also grepped for -fdirect-access-
external-data in the kernel building system but I've not found any
match.
Are you using a different version of Clang, or maybe Clang has some
configuration-time option to make -fdirect-access-external-data the
default?
Note that to translate a TU for a normal (dynamically-linked user-space)
executable on LoongArch Linux, -fdirect-access-external-data should not
be used (because copy relocation is now considered a bad idea and we'll
not support it for a new architecture). Fangrui?
-fdirect-access-external-data can be used in KBUILD_CFLAGS_KERNEL for
avoiding GOT in the main kernel image, OTOH.
--
Xi Ruoyao <xry111@xxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
