Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

Josh Poimboeuf
2021-11-04T21:49:44Z

On Thu, Nov 04, 2021 at 05:47:36PM +0100, Peter Zijlstra wrote:
> +asm(
> +" .macro extable_type_reg type:req reg:req\n"
> +" .set regnr, 0\n"
> +" .irp rs,rax,rcx,rdx,rbx,rsp,rbp,rsi,rdi,r8,r9,r10,r11,r12,r13,r14,r15\n"
> +" .ifc \\reg, %\\rs\n"
> +" .long \\type + (regnr << 8)\n"
> +" .endif\n"
> +" .set regnr, regnr+1\n"
> +" .endr\n"
> +" .set regnr, 0\n"
> +" .irp rs,eax,ecx,edx,ebx,esp,ebp,esi,edi,r8d,r9d,r10d,r11d,r12d,r13d,r14d,r15d\n"
> +" .ifc \\reg, %\\rs\n"
> +" .long \\type + (regnr << 8)\n"
> +" .endif\n"
> +" .set regnr, regnr+1\n"
> +" .endr\n"
> +" .endm\n"
> +);

How about some error checking to detect a typo, or a forgotten '%':

diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index 5d0ff8c60983..95bb23082b87 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -154,9 +154,11 @@

asm(
" .macro extable_type_reg type:req reg:req\n"
+" .set found, 0\n"
" .set regnr, 0\n"
" .irp rs,rax,rcx,rdx,rbx,rsp,rbp,rsi,rdi,r8,r9,r10,r11,r12,r13,r14,r15\n"
" .ifc \\reg, %\\rs\n"
+" .set found, found+1\n"
" .long \\type + (regnr << 8)\n"
" .endif\n"
" .set regnr, regnr+1\n"
@@ -164,10 +166,14 @@ asm(
" .set regnr, 0\n"
" .irp rs,eax,ecx,edx,ebx,esp,ebp,esi,edi,r8d,r9d,r10d,r11d,r12d,r13d,r14d,r15d\n"
" .ifc \\reg, %\\rs\n"
+" .set found, found+1\n"
" .long \\type + (regnr << 8)\n"
" .endif\n"
" .set regnr, regnr+1\n"
" .endr\n"
+" .if (found != 1)\n"
+" .error \"extable_type_reg: bad register argument\"\n"
+" .endif\n"
" .endm\n"
);

> +#define EX_FLAG_CLR_AX EX_TYPE_FLAG(1)
> +#define EX_FLAG_CLR_DX EX_TYPE_FLAG(2)
> +#define EX_FLAG_CLR_AX_DX EX_TYPE_FLAG(3)

I'd like to buy two vowels: CL̲E̲AR

(I hope that Wheel of Fortune reference isn't too US-centric.)

> +static inline unsigned long *pt_regs_nr(struct pt_regs *regs, int nr)
> +{
> + /* because having pt_regs in machine order was too much to ask */
> + switch (nr) {
> + case 0: return &regs->ax;
> + case 1: return &regs->cx;
> + case 2: return &regs->dx;
> + case 3: return &regs->bx;
> + case 4: return &regs->sp;
> + case 5: return &regs->bp;
> + case 6: return &regs->si;
> + case 7: return &regs->di;
> +#ifdef CONFIG_X86_64
> + case 8: return &regs->r8;
> + case 9: return &regs->r9;
> + case 10: return &regs->r10;
> + case 11: return &regs->r11;
> + case 12: return &regs->r12;
> + case 13: return &regs->r13;
> + case 14: return &regs->r14;
> + case 15: return &regs->r15;
> +#endif
> + default: return NULL;
> + }
> +}

Instead of all this craziness, why not just admit defeat and put them in
pt_regs order in the 'extable_type_reg' macro?

> +static bool ex_handler_imm_reg(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int reg, int imm)
> +{
> + *pt_regs_nr(regs, reg) = (long)imm;
> + return ex_handler_default(fixup, regs);
> +}
> +
> +#define EX_TYPE_MASK 0x000000FF
> +#define EX_REG_MASK 0x00000F00
> +#define EX_FLAG_MASK 0x0000F000
> +#define EX_IMM_MASK 0xFFFF0000

To avoid mismatches these should probably be in the header file next to
EX_TYPE_*_SHIFT?

> +
> int ex_get_fixup_type(unsigned long ip)
> {
> const struct exception_table_entry *e = search_exception_tables(ip);
>
> - return e ? e->type : EX_TYPE_NONE;
> + return e ? FIELD_GET(EX_TYPE_MASK, e->type) : EX_TYPE_NONE;

Maybe the 'type' field should be renamed, to better represent its new
use, and to try to discourage direct access. Not that I have any good
ideas. Some not-so-good ideas: "handler", "flags", "_type".

--
Josh