Re: [PATCH v4 3/5] x86/syscall: Mark exit[_group] syscall handlers __noreturn

From: Paul McKenney
Date: Sun Apr 21 2024 - 17:47:44 EST

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In reply to: Paul McKenney: "Re: [PATCH v4 3/5] x86/syscall: Mark exit[_group] syscall handlers __noreturn"
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And this definitely helped, thank you!

However, this one still remains:

vmlinux.o: warning: objtool: ia32_sys_call+0x29b6:
__ia32_sys_exit_group() is missing a __noreturn annotation

Please see below for my diffs against next-20240419, in case I messed
something up.

I attached a copy as well, given that I am away from mutt, hence using
gmail directly.

Thanx, Paul

-----------------------------------------------------

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 6de50b80702e6..9810ba2857a5c 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -49,7 +49,10 @@ static __always_inline bool do_syscall_x64(struct
pt_regs *regs, int nr)

if (likely(unr < NR_syscalls)) {
unr = array_index_nospec(unr, NR_syscalls);
- regs->ax = x64_sys_call(regs, unr);
+ if (likely(cpu_feature_enabled(X86_FEATURE_INDIRECT_BRANCH_OK)))
+ regs->ax = sys_call_table[unr](regs);
+ else
+ regs->ax = x64_sys_call(regs, unr);
return true;
}
return false;
@@ -66,7 +69,10 @@ static __always_inline bool do_syscall_x32(struct
pt_regs *regs, int nr)

if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) {
xnr = array_index_nospec(xnr, X32_NR_syscalls);
- regs->ax = x32_sys_call(regs, xnr);
+ if (likely(cpu_feature_enabled(X86_FEATURE_INDIRECT_BRANCH_OK)))
+ regs->ax = x32_sys_call_table[xnr](regs);
+ else
+ regs->ax = x32_sys_call(regs, xnr);
return true;
}
return false;
@@ -162,7 +168,10 @@ static __always_inline void
do_syscall_32_irqs_on(struct pt_regs *regs, int nr)

if (likely(unr < IA32_NR_syscalls)) {
unr = array_index_nospec(unr, IA32_NR_syscalls);
- regs->ax = ia32_sys_call(regs, unr);
+ if (likely(cpu_feature_enabled(X86_FEATURE_INDIRECT_BRANCH_OK)))
+ regs->ax = ia32_sys_call_table[unr](regs);
+ else
+ regs->ax = ia32_sys_call(regs, unr);
} else if (nr != -1) {
regs->ax = __ia32_sys_ni_syscall(regs);
}
diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
index c2235bae17ef6..aab31760b4e3e 100644
--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -14,25 +14,16 @@
#endif

#define __SYSCALL(nr, sym) extern long __ia32_##sym(const struct pt_regs *);
-
#include <asm/syscalls_32.h>
#undef __SYSCALL

-/*
- * The sys_call_table[] is no longer used for system calls, but
- * kernel/trace/trace_syscalls.c still wants to know the system
- * call address.
- */
-#ifdef CONFIG_X86_32
#define __SYSCALL(nr, sym) __ia32_##sym,
-const sys_call_ptr_t sys_call_table[] = {
+const sys_call_ptr_t ia32_sys_call_table[] = {
#include <asm/syscalls_32.h>
};
#undef __SYSCALL
-#endif

#define __SYSCALL(nr, sym) case nr: return __ia32_##sym(regs);
-
long ia32_sys_call(const struct pt_regs *regs, unsigned int nr)
{
switch (nr) {
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index 33b3f09e6f151..ff36a993a07e0 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -8,14 +8,13 @@
#include <asm/syscall.h>

#define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn
__x64_##sym(const struct pt_regs *);
#include <asm/syscalls_64.h>
#undef __SYSCALL

-/*
- * The sys_call_table[] is no longer used for system calls, but
- * kernel/trace/trace_syscalls.c still wants to know the system
- * call address.
- */
+#undef __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
+
#define __SYSCALL(nr, sym) __x64_##sym,
const sys_call_ptr_t sys_call_table[] = {
#include <asm/syscalls_64.h>
@@ -23,7 +22,6 @@ const sys_call_ptr_t sys_call_table[] = {
#undef __SYSCALL

#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
-
long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
{
switch (nr) {
diff --git a/arch/x86/entry/syscall_x32.c b/arch/x86/entry/syscall_x32.c
index 03de4a9321318..4221ecce6e689 100644
--- a/arch/x86/entry/syscall_x32.c
+++ b/arch/x86/entry/syscall_x32.c
@@ -8,11 +8,20 @@
#include <asm/syscall.h>

#define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn
__x64_##sym(const struct pt_regs *);
#include <asm/syscalls_x32.h>
#undef __SYSCALL

-#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
+#undef __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
+
+#define __SYSCALL(nr, sym) __x64_##sym,
+const sys_call_ptr_t x32_sys_call_table[] = {
+#include <asm/syscalls_x32.h>
+};
+#undef __SYSCALL

+#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
{
switch (nr) {
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl
b/arch/x86/entry/syscalls/syscall_64.tbl
index a396f6e6ab5bf..7ec68d94eb593 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -2,7 +2,7 @@
# 64-bit system call numbers and entry vectors
#
# The format is:
-# <number> <abi> <name> <entry point>
+# <number> <abi> <name> <entry point> [0 noreturn]
#
# The __x64_sys_*() stubs are created on-the-fly for sys_*() system calls
#
@@ -68,7 +68,7 @@
57 common fork sys_fork
58 common vfork sys_vfork
59 64 execve sys_execve
-60 common exit sys_exit
+60 common exit sys_exit 0 noreturn
61 common wait4 sys_wait4
62 common kill sys_kill
63 common uname sys_newuname
@@ -239,7 +239,7 @@
228 common clock_gettime sys_clock_gettime
229 common clock_getres sys_clock_getres
230 common clock_nanosleep sys_clock_nanosleep
-231 common exit_group sys_exit_group
+231 common exit_group sys_exit_group 0 noreturn
232 common epoll_wait sys_epoll_wait
233 common epoll_ctl sys_epoll_ctl
234 common tgkill sys_tgkill
diff --git a/arch/x86/include/asm/cpufeatures.h
b/arch/x86/include/asm/cpufeatures.h
index 3c7434329661c..d64b0a5291f10 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -470,6 +470,7 @@
#define X86_FEATURE_BHI_CTRL (21*32+ 2) /* "" BHI_DIS_S HW control available */
#define X86_FEATURE_CLEAR_BHB_HW (21*32+ 3) /* "" BHI_DIS_S HW
control enabled */
#define X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT (21*32+ 4) /* "" Clear
branch history at vmexit using SW loop */
+#define X86_FEATURE_INDIRECT_BRANCH_OK (21*32+ 5) /* "" It's OK to
use indirect branches */

/*
* BUG word(s)
diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index 2fc7bc3863ff6..dfb59521244c2 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -16,14 +16,20 @@
#include <asm/thread_info.h> /* for TS_COMPAT */
#include <asm/unistd.h>

-/* This is used purely for kernel/trace/trace_syscalls.c */
typedef long (*sys_call_ptr_t)(const struct pt_regs *);
extern const sys_call_ptr_t sys_call_table[];

+#if defined(CONFIG_X86_32)
+#define ia32_sys_call_table sys_call_table
+#else
/*
* These may not exist, but still put the prototypes in so we
* can use IS_ENABLED().
*/
+extern const sys_call_ptr_t ia32_sys_call_table[];
+extern const sys_call_ptr_t x32_sys_call_table[];
+#endif
+
extern long ia32_sys_call(const struct pt_regs *, unsigned int nr);
extern long x32_sys_call(const struct pt_regs *, unsigned int nr);
extern long x64_sys_call(const struct pt_regs *, unsigned int nr);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index ab18185894dfd..5fca46c78daf2 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1665,6 +1665,12 @@ static void __init bhi_select_mitigation(void)
if (!IS_ENABLED(CONFIG_X86_64))
return;

+ /*
+ * There's no HW mitigation in place. Mark indirect branches as
+ * "not OK".
+ */
+ setup_clear_cpu_cap(X86_FEATURE_INDIRECT_BRANCH_OK);
+
/* Mitigate KVM by default */
setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
@@ -1679,6 +1685,28 @@ static void __init spectre_v2_select_mitigation(void)
enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
enum spectre_v2_mitigation mode = SPECTRE_V2_NONE;

+ /*
+ * X86_FEATURE_INDIRECT_BRANCH_OK indicates that indirect calls are
+ * "OK" to use due to (at least) one of the following being true:
+ *
+ * - the CPU isn't vulnerable to Spectre v2, BHI, etc;
+ *
+ * - a HW mitigation is in place (e.g., IBRS, eIBRS+BHI_DIS_S); or
+ *
+ * - the user disabled mitigations.
+ *
+ * Clearing the bit enables certain indirect branch "easy targets" [*]
+ * to be converted to a series of direct branches.
+ *
+ * Assume innocence until proven guilty: set it now and clear it later
+ * if/when needed.
+ *
+ * [*] The closer the indirect branch is to kernel entry, and the more
+ * user-controlled registers there are, the easier target it may be
+ * for future Spectre v2 variants.
+ */
+ setup_force_cpu_cap(X86_FEATURE_INDIRECT_BRANCH_OK);
+
/*
* If the CPU is not affected and the command line mode is NONE or AUTO
* then nothing to do.
@@ -1765,11 +1793,16 @@ static void __init spectre_v2_select_mitigation(void)
break;

case SPECTRE_V2_LFENCE:
+ setup_clear_cpu_cap(X86_FEATURE_INDIRECT_BRANCH_OK);
+ fallthrough;
case SPECTRE_V2_EIBRS_LFENCE:
setup_force_cpu_cap(X86_FEATURE_RETPOLINE_LFENCE);
- fallthrough;
+ setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
+ break;

case SPECTRE_V2_RETPOLINE:
+ setup_clear_cpu_cap(X86_FEATURE_INDIRECT_BRANCH_OK);
+ fallthrough;
case SPECTRE_V2_EIBRS_RETPOLINE:
setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
break;
diff --git a/arch/x86/um/sys_call_table_32.c b/arch/x86/um/sys_call_table_32.c
index 89df5d89d6640..c7d4bf955d2ba 100644
--- a/arch/x86/um/sys_call_table_32.c
+++ b/arch/x86/um/sys_call_table_32.c
@@ -24,6 +24,7 @@
#define __SYSCALL_WITH_COMPAT(nr, native, compat) __SYSCALL(nr, native)

#define __SYSCALL(nr, sym) extern asmlinkage long sym(unsigned long,
unsigned long, unsigned long, unsigned long, unsigned long, unsigned
long);
+#define __SYSCALL_NORETURN __SYSCALL
#include <asm/syscalls_32.h>

#undef __SYSCALL
diff --git a/arch/x86/um/sys_call_table_64.c b/arch/x86/um/sys_call_table_64.c
index b0b4cfd2308c8..4760c40ae5cd0 100644
--- a/arch/x86/um/sys_call_table_64.c
+++ b/arch/x86/um/sys_call_table_64.c
@@ -19,6 +19,7 @@
#define sys_ioperm sys_ni_syscall

#define __SYSCALL(nr, sym) extern asmlinkage long sym(unsigned long,
unsigned long, unsigned long, unsigned long, unsigned long, unsigned
long);
+#define __SYSCALL_NORETURN __SYSCALL
#include <asm/syscalls_64.h>

#undef __SYSCALL
diff --git a/scripts/syscalltbl.sh b/scripts/syscalltbl.sh
index 6abe143889ef6..16487d47e06a3 100755
--- a/scripts/syscalltbl.sh
+++ b/scripts/syscalltbl.sh
@@ -54,7 +54,7 @@ nxt=0

grep -E "^[0-9]+[[:space:]]+$abis" "$infile" | {

- while read nr abi name native compat ; do
+ while read nr abi name native compat noreturn; do

if [ $nxt -gt $nr ]; then
echo "error: $infile: syscall table is not sorted or duplicates the
same syscall number" >&2
@@ -66,7 +66,9 @@ grep -E "^[0-9]+[[:space:]]+$abis" "$infile" | {
nxt=$((nxt + 1))
done

- if [ -n "$compat" ]; then
+ if [ -n "$noreturn" ]; then
+ echo "__SYSCALL_NORETURN($nr, $native)"
+ elif [ -n "$compat" ]; then
echo "__SYSCALL_WITH_COMPAT($nr, $native, $compat)"
elif [ -n "$native" ]; then
echo "__SYSCALL($nr, $native)"
diff --git a/tools/objtool/noreturns.h b/tools/objtool/noreturns.h
index 7ebf29c911849..1e8141ef1b15d 100644
--- a/tools/objtool/noreturns.h
+++ b/tools/objtool/noreturns.h
@@ -7,12 +7,16 @@
* Yes, this is unfortunate. A better solution is in the works.
*/
NORETURN(__fortify_panic)
+NORETURN(__ia32_sys_exit)
+NORETURN(__ia32_sys_exit_group)
NORETURN(__kunit_abort)
NORETURN(__module_put_and_kthread_exit)
NORETURN(__reiserfs_panic)
NORETURN(__stack_chk_fail)
NORETURN(__tdx_hypercall_failed)
NORETURN(__ubsan_handle_builtin_unreachable)
+NORETURN(__x64_sys_exit)
+NORETURN(__x64_sys_exit_group)
NORETURN(arch_cpu_idle_dead)
NORETURN(bch2_trans_in_restart_error)
NORETURN(bch2_trans_restart_error)

On Sun, Apr 21, 2024 at 1:40 PM Paul McKenney <paulmckrcu@xxxxxxxxx> wrote:
>
> They apply fine as is, so I have started tests with that pair of patches.
>
> Thanx, Paul
>
> On Sat, Apr 20, 2024 at 10:25 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > On Sat, Apr 20, 2024 at 06:58:58AM -0700, Paul E. McKenney wrote:
> > > On Fri, Apr 19, 2024 at 02:09:49PM -0700, Josh Poimboeuf wrote:
> > > > The direct-call syscall dispatch functions don't know that the exit()
> > > > and exit_group() syscall handlers don't return. As a result the call
> > > > sites aren't optimized accordingly.
> > > >
> > > > Fix that by marking those exit syscall declarations as __noreturn.
> > > >
> > > > Fixes the following warnings:
> > > >
> > > > vmlinux.o: warning: objtool: x64_sys_call+0x2804: __x64_sys_exit() is missing a __noreturn annotation
> > > > vmlinux.o: warning: objtool: ia32_sys_call+0x29b6: __ia32_sys_exit_group() is missing a __noreturn annotation
> > > >
> > > > Fixes: 7390db8aea0d ("x86/bhi: Add support for clearing branch history at syscall entry")
> > > > Reported-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > > Closes: https://lkml.kernel.org/lkml/6dba9b32-db2c-4e6d-9500-7a08852f17a3@paulmck-laptop
> > > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > >
> > > Looks good, but it does not apply on top of current -next and I don't
> > > trust myself to hand-apply it (something about having just got off of
> > > a flight across the big pond).
> > >
> > > Could you please let me know what else do I need to pull in to be able
> > > to cleanly apply this one?
> >
> > This patch has a dependency on an earlier patch in the set:
> >
> > https://lkml.kernel.org/lkml/982d05a2f669140f26500bee643011896d6610941713559768.git.jpoimboe@xxxxxxxxxx
> >
> > Though I think it's not a hard dependency and I could reverse the order
> > of the patches if needed.
> >
> > --
> > Josh

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Next message: Stephen Rothwell: "linux-next: duplicate patch in the mm-hotfixes tree"
Previous message: Jan Dakinevich: "Re: [RFC PATCH v3 4/6] dt-bindings: clock: meson: document A1 SoC audio clock controller driver"
In reply to: Paul McKenney: "Re: [PATCH v4 3/5] x86/syscall: Mark exit[_group] syscall handlers __noreturn"
Next in thread: Josh Poimboeuf: "[PATCH v4 4/5] x86/bugs: Remove duplicate Spectre cmdline option descriptions"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]