Re: PROPOSAL: Extend inline asm syntax with size spec

Borislav Petkov
2018-10-13T19:33:46Z

Ok,

with Segher's help I've been playing with his patch ontop of bleeding
edge gcc 9 and here are my observations. Please double-check me for
booboos so that they can be addressed while there's time.

So here's what I see ontop of 4.19-rc7:

First marked the alternative asm() as inline and undeffed the "inline"
keyword. I need to do that because of the funky games we do with
"inline" redefinitions in include/linux/compiler_types.h.

And Segher hinted at either doing:

asm volatile inline(...

or

asm volatile __inline__(...

but both "inline" variants are defined as macros in that file.

Which means we either need to #undef inline before using it in asm() or
come up with something cleverer.

Anyway, did this:

---
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 4cd6a3b71824..7c0639087da7 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -165,11 +165,13 @@ static inline int alternatives_text_reserved(void *start, void *end)
* For non barrier like inlines please define new variants
* without volatile and memory clobber.
*/
+
+#undef inline
#define alternative(oldinstr, newinstr, feature) \
- asm volatile (ALTERNATIVE(oldinstr, newinstr, feature) : : : "memory")
+ asm volatile inline(ALTERNATIVE(oldinstr, newinstr, feature) : : : "memory")

#define alternative_2(oldinstr, newinstr1, feature1, newinstr2, feature2) \
- asm volatile(ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2) ::: "memory")
+ asm volatile inline(ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2) ::: "memory")

/*
* Alternative inline assembly with input.
---

Build failed at link time with:

arch/x86/boot/compressed/cmdline.o: In function `native_save_fl':
cmdline.c:(.text+0x0): multiple definition of `native_save_fl'
arch/x86/boot/compressed/misc.o:misc.c:(.text+0x1e0): first defined here
arch/x86/boot/compressed/cmdline.o: In function `native_restore_fl':
cmdline.c:(.text+0x10): multiple definition of `native_restore_fl'
arch/x86/boot/compressed/misc.o:misc.c:(.text+0x1f0): first defined here
arch/x86/boot/compressed/error.o: In function `native_save_fl':
error.c:(.text+0x0): multiple definition of `native_save_fl'

which I had to fix with this:

---
diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index 15450a675031..0d772598c37c 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -14,8 +14,7 @@
*/

/* Declaration required for gcc < 4.9 to prevent -Werror=missing-prototypes */
-extern inline unsigned long native_save_fl(void);
-extern inline unsigned long native_save_fl(void)
+static inline unsigned long native_save_fl(void)
{
unsigned long flags;

@@ -33,8 +32,7 @@ ex
---

That "extern inline" declaration looks fishy to me anyway, maybe not really
needed ? Apparently, gcc < 4.9 complains with -Werror=missing-prototypes...

Then the build worked and the results look like this:

text data bss dec hex filename
17287384 5040656 2019532 24347572 17383b4 vmlinux-before
17288020 5040656 2015436 24344112 1737630 vmlinux-2nd-version

so some inlining must be happening.

Then I did this:

---
diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index 9c5606d88f61..a0170344cf08 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -20,7 +20,7 @@ unsigned long __clear_user(void __user *addr, unsigned long size)
/* no memory constraint because it doesn't change any memory gcc knows
about */
stac();
- asm volatile(
+ asm volatile inline(
" testq %[size8],%[size8]\n"
" jz 4f\n"
"0: movq $0,(%[dst])\n"
---

to force inlining of a somewhat bigger asm() statement. And yap, more
got inlined:

text data bss dec hex filename
17287384 5040656 2019532 24347572 17383b4 vmlinux-before
17288020 5040656 2015436 24344112 1737630 vmlinux-2nd-version
17288076 5040656 2015436 24344168 1737668 vmlinux-2nd-version__clear_user

so more stuff gets inlined.

Looking at the asm output, it had before:

---
clear_user:
# ./arch/x86/include/asm/current.h:15: return this_cpu_read_stable(current_task);
#APP
# 15 "./arch/x86/include/asm/current.h" 1
movq %gs:current_task,%rax #, pfo_ret__
# 0 "" 2
# arch/x86/lib/usercopy_64.c:51: if (access_ok(VERIFY_WRITE, to, n))
#NO_APP
movq 2520(%rax), %rdx # pfo_ret___12->thread.addr_limit.seg, _1
movq %rdi, %rax # to, tmp93
addq %rsi, %rax # n, tmp93
jc .L3 #,
# arch/x86/lib/usercopy_64.c:51: if (access_ok(VERIFY_WRITE, to, n))
cmpq %rax, %rdx # tmp93, _1
jb .L3 #,
# arch/x86/lib/usercopy_64.c:52: return __clear_user(to, n);
jmp __clear_user #
---

note the JMP to __clear_user. After marking the asm() in __clear_user() as
inline, clear_user() inlines __clear_user() directly:

---
clear_user:
# ./arch/x86/include/asm/current.h:15: return this_cpu_read_stable(current_task);
#APP
# 15 "./arch/x86/include/asm/current.h" 1
movq %gs:current_task,%rax #, pfo_ret__
# 0 "" 2
# arch/x86/lib/usercopy_64.c:51: if (access_ok(VERIFY_WRITE, to, n))
#NO_APP
movq 2520(%rax), %rdx # pfo_ret___12->thread.addr_limit.seg, _1
movq %rdi, %rax # to, tmp95
addq %rsi, %rax # n, tmp95
jc .L8 #,
# arch/x86/lib/usercopy_64.c:51: if (access_ok(VERIFY_WRITE, to, n))
cmpq %rax, %rdx # tmp95, _1
jb .L8 #,
# ./arch/x86/include/asm/smap.h:58: alternative("", __stringify(__ASM_STAC), X86_FEATURE_SMAP);
...

this last line is the stac() macro which gets inlined due to the
alternative() inlined macro above.

So I guess this all looks like what we wanted.

And if this lands in gcc9, we would need to do a asm_volatile() macro
which is defined differently based on the compiler used.

Thoughts, suggestions, etc are most welcome.

Thx.

--
Regards/Gruss,
Boris.

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