On Tue, Oct 15, 2019 at 12:41 AM Thomas Huth <thuth@xxxxxxxxxx> wrote:
>
> On 13/10/2019 09.18, Bill Wendling wrote:
> > Two global objects can't have the same address in C. Clang uses this
> > fact to omit the check on the first iteration of the loop in
> > check_exception_table.
> >
> > Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> > ---
> > lib/x86/desc.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/x86/desc.c b/lib/x86/desc.c
> > index 451f504..cfc449f 100644
> > --- a/lib/x86/desc.c
> > +++ b/lib/x86/desc.c
> > @@ -41,7 +41,7 @@ struct ex_record {
> > unsigned long handler;
> > };
> >
> > -extern struct ex_record exception_table_start, exception_table_end;
> > +extern struct ex_record exception_table_start, *exception_table_end;
> >
> > static const char* exception_mnemonic(int vector)
> > {
> > @@ -113,7 +113,7 @@ static void check_exception_table(struct ex_regs *regs)
> > (((regs->rflags >> 16) & 1) << 8);
> > asm("mov %0, %%gs:4" : : "r"(ex_val));
> >
> > - for (ex = &exception_table_start; ex != &exception_table_end; ++ex) {
> > + for (ex = &exception_table_start; ex != (void*)&exception_table_end; ++ex) {
> > if (ex->rip == regs->rip) {
> > regs->rip = ex->handler;
> > return;
> >
>
> That looks like quite an ugly hack to me - and if clang gets a little
> bit smarter, I'm pretty sure it will optimize this away, too.
>
I agree. I was thinking about this actually and this patch looks like
it'll work. I'll generate a new version of the patches.
diff --git a/lib/x86/desc.c b/lib/x86/desc.c
index 451f504..4002203 100644
--- a/lib/x86/desc.c
+++ b/lib/x86/desc.c
@@ -113,7 +113,7 @@ static void check_exception_table(struct ex_regs *regs)
(((regs->rflags >> 16) & 1) << 8);
asm("mov %0, %%gs:4" : : "r"(ex_val));
- for (ex = &exception_table_start; ex != &exception_table_end; ++ex) {
+ for (ex = &exception_table_start; ex < &exception_table_end; ++ex) {
if (ex->rip == regs->rip) {
regs->rip = ex->handler;
return;
> Could you please check if something like this works, too:
>
> diff --git a/lib/x86/desc.c b/lib/x86/desc.c
> index 451f504..0e9779b 100644
> --- a/lib/x86/desc.c
> +++ b/lib/x86/desc.c
> @@ -41,7 +41,8 @@ struct ex_record {
> unsigned long handler;
> };
>
> -extern struct ex_record exception_table_start, exception_table_end;
> +extern struct ex_record exception_table_start;
> +extern long exception_table_size;
>
> static const char* exception_mnemonic(int vector)
> {
> @@ -108,12 +109,13 @@ static void check_exception_table(struct ex_regs
> *regs)
> {
> struct ex_record *ex;
> unsigned ex_val;
> + int cnt = exception_table_size / sizeof(struct ex_record);
>
> ex_val = regs->vector | (regs->error_code << 16) |
> (((regs->rflags >> 16) & 1) << 8);
> asm("mov %0, %%gs:4" : : "r"(ex_val));
>
> - for (ex = &exception_table_start; ex != &exception_table_end; ++ex) {
> + for (ex = &exception_table_start; cnt-- > 0; ++ex) {
> if (ex->rip == regs->rip) {
> regs->rip = ex->handler;
> return;
> diff --git a/x86/flat.lds b/x86/flat.lds
> index a278b56..108fad5 100644
> --- a/x86/flat.lds
> +++ b/x86/flat.lds
> @@ -9,6 +9,7 @@ SECTIONS
> exception_table_start = .;
> *(.data.ex)
> exception_table_end = .;
> + exception_table_size = exception_table_end -
> exception_table_start;
> }
> . = ALIGN(16);
> .rodata : { *(.rodata) }
>
> Thomas
