x86/extable: Rework the exception table mechanics

The exception table entries contain the instruction address, the fixup
address and the handler address. All addresses are relative. Storing the
handler address has a few downsides:

 1) Most handlers need to be exported

 2) Handlers can be defined everywhere and there is no overview about the
    handler types

 3) MCE needs to check the handler type to decide whether an in kernel #MC
    can be recovered. The functionality of the handler itself is not in any
    way special, but for these checks there need to be separate functions
    which in the worst case have to be exported.

    Some of these 'recoverable' exception fixups are pretty obscure and
    just reuse some other handler to spare code. That obfuscates e.g. the
    #MC safe copy functions. Cleaning that up would require more handlers
    and exports

Rework the exception fixup mechanics by storing a fixup type number instead
of the handler address and invoke the proper handler for each fixup
type. Also teach the extable sort to leave the type field alone.

This makes most handlers static except for special cases like the MCE
MSR fixup and the BPF fixup. This allows to add more types for cleaning up
the obscure places without adding more handler code and exports.

There is a marginal code size reduction for a production config and it
removes _eight_ exported symbols.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lkml.kernel.org/r/20210908132525.211958725@linutronix.de

1 files changed, 2 insertions, 9 deletions

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 0fe6aacef3db..703dc6eaf4c7 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -827,9 +827,7 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
 	return 0;
 }
 
-static bool ex_handler_bpf(const struct exception_table_entry *x,
-			   struct pt_regs *regs, int trapnr,
-			   unsigned long error_code, unsigned long fault_addr)
+bool ex_handler_bpf(const struct exception_table_entry *x, struct pt_regs *regs)
 {
 	u32 reg = x->fixup >> 8;
 
@@ -1313,12 +1311,7 @@ st:			if (is_imm8(insn->off))
 				}
 				ex->insn = delta;
 
-				delta = (u8 *)ex_handler_bpf - (u8 *)&ex->handler;
-				if (!is_simm32(delta)) {
-					pr_err("extable->handler doesn't fit into 32-bit\n");
-					return -EFAULT;
-				}
-				ex->handler = delta;
+				ex->type = EX_TYPE_BPF;
 
 				if (dst_reg > BPF_REG_9) {
 					pr_err("verifier error\n");