objtool: Support stack layout changes in alternatives

The ORC unwinder showed a warning [1] which revealed the stack layout
didn't match what was expected. The problem was that paravirt patching
had replaced "CALL *pv_ops.irq.save_fl" with "PUSHF;POP". That changed
the stack layout between the PUSHF and the POP, so unwinding from an
interrupt which occurred between those two instructions would fail.

Part of the agreed upon solution was to rework the custom paravirt
patching code to use alternatives instead, since objtool already knows
how to read alternatives (and converging runtime patching infrastructure
is always a good thing anyway). But the main problem still remains,
which is that runtime patching can change the stack layout.

Making stack layout changes in alternatives was disallowed with commit
7117f16bf460 ("objtool: Fix ORC vs alternatives"), but now that paravirt
is going to be doing it, it needs to be supported.

One way to do so would be to modify the ORC table when the code gets
patched. But ORC is simple -- a good thing! -- and it's best to leave
it alone.

Instead, support stack layout changes by "flattening" all possible stack
states (CFI) from parallel alternative code streams into a single set of
linear states. The only necessary limitation is that CFI conflicts are
disallowed at all possible instruction boundaries.

For example, this scenario is allowed:

Alt1 Alt2 Alt3

0x00 CALL *pv_ops.save_fl CALL xen_save_fl PUSHF
0x01 POP %RAX
0x02 NOP
...
0x05 NOP
...
0x07 <insn>

The unwind information for offset-0x00 is identical for all 3
alternatives. Similarly offset-0x05 and higher also are identical (and
the same as 0x00). However offset-0x01 has deviating CFI, but that is
only relevant for Alt3, neither of the other alternative instruction
streams will ever hit that offset.

This scenario is NOT allowed:

Alt1 Alt2

0x00 CALL *pv_ops.save_fl PUSHF
0x01 NOP6
...
0x07 NOP POP %RAX

The problem here is that offset-0x7, which is an instruction boundary in
both possible instruction patch streams, has two conflicting stack
layouts.

[ The above examples were stolen from Peter Zijlstra. ]

The new flattened CFI array is used both for the detection of conflicts
(like the second example above) and the generation of linear ORC
entries.

BTW, another benefit of these changes is that, thanks to some related
cleanups (new fake nops and alt_group struct) objtool can finally be rid
of fake jumps, which were a constant source of headaches.

[1] https://lkml.kernel.org/r/20201111170536.arx2zbn4ngvjoov7@treble

Cc: Shinichiro Kawasaki <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>

tools/objtool/Documentation/stack-validation.txt

···
       function tracing inserts additional calls, which is not obvious from the
       sources).
 
-10. file.o: warning: func()+0x5c: alternative modifies stack
+10. file.o: warning: func()+0x5c: stack layout conflict in alternatives
 
-    This means that an alternative includes instructions that modify the
-    stack. The problem is that there is only one ORC unwind table, this means
-    that the ORC unwind entries must be valid for each of the alternatives.
-    The easiest way to enforce this is to ensure alternatives do not contain
-    any ORC entries, which in turn implies the above constraint.
+    This means that in the use of the alternative() or ALTERNATIVE()
+    macro, the code paths have conflicting modifications to the stack.
+    The problem is that there is only one ORC unwind table, which means
+    that the ORC unwind entries must be consistent for all possible
+    instruction boundaries regardless of which code has been patched.
+    This limitation can be overcome by massaging the alternatives with
+    NOPs to shift the stack changes around so they no longer conflict.
 
 11. file.o: warning: unannotated intra-function call
 

tools/objtool/check.c

···
 #include <linux/kernel.h>
 #include <linux/static_call_types.h>
 
-#define FAKE_JUMP_OFFSET -1
-
 struct alternative {
 	struct list_head list;
 	struct instruction *insn;
···
 		if (!is_static_jump(insn))
 			continue;
 
-		if (insn->offset == FAKE_JUMP_OFFSET)
-			continue;
-
 		reloc = find_reloc_by_dest_range(file->elf, insn->sec,
 					       insn->offset, insn->len);
 		if (!reloc) {
···
 }
 
 /*
- * The .alternatives section requires some extra special care, over and above
- * what other special sections require:
- *
- * 1. Because alternatives are patched in-place, we need to insert a fake jump
- *    instruction at the end so that validate_branch() skips all the original
- *    replaced instructions when validating the new instruction path.
- *
- * 2. An added wrinkle is that the new instruction length might be zero.  In
- *    that case the old instructions are replaced with noops.  We simulate that
- *    by creating a fake jump as the only new instruction.
- *
- * 3. In some cases, the alternative section includes an instruction which
- *    conditionally jumps to the _end_ of the entry.  We have to modify these
- *    jumps' destinations to point back to .text rather than the end of the
- *    entry in .altinstr_replacement.
+ * The .alternatives section requires some extra special care over and above
+ * other special sections because alternatives are patched in place.
  */
 static int handle_group_alt(struct objtool_file *file,
 			    struct special_alt *special_alt,
 			    struct instruction *orig_insn,
 			    struct instruction **new_insn)
 {
-	struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = NULL;
+	struct instruction *last_orig_insn, *last_new_insn = NULL, *insn, *nop = NULL;
 	struct alt_group *orig_alt_group, *new_alt_group;
 	unsigned long dest_off;
 
···
 		WARN("malloc failed");
 		return -1;
 	}
+	orig_alt_group->cfi = calloc(special_alt->orig_len,
+				     sizeof(struct cfi_state *));
+	if (!orig_alt_group->cfi) {
+		WARN("calloc failed");
+		return -1;
+	}
+
 	last_orig_insn = NULL;
 	insn = orig_insn;
 	sec_for_each_insn_from(file, insn) {
···
 	orig_alt_group->first_insn = orig_insn;
 	orig_alt_group->last_insn = last_orig_insn;
 
-	if (next_insn_same_sec(file, last_orig_insn)) {
-		fake_jump = malloc(sizeof(*fake_jump));
-		if (!fake_jump) {
-			WARN("malloc failed");
-			return -1;
-		}
-		memset(fake_jump, 0, sizeof(*fake_jump));
-		INIT_LIST_HEAD(&fake_jump->alts);
-		INIT_LIST_HEAD(&fake_jump->stack_ops);
-		init_cfi_state(&fake_jump->cfi);
-
-		fake_jump->sec = special_alt->new_sec;
-		fake_jump->offset = FAKE_JUMP_OFFSET;
-		fake_jump->type = INSN_JUMP_UNCONDITIONAL;
-		fake_jump->jump_dest = list_next_entry(last_orig_insn, list);
-		fake_jump->func = orig_insn->func;
-	}
-
-	if (!special_alt->new_len) {
-		if (!fake_jump) {
-			WARN("%s: empty alternative at end of section",
-			     special_alt->orig_sec->name);
-			return -1;
-		}
-
-		*new_insn = fake_jump;
-		return 0;
-	}
 
 	new_alt_group = malloc(sizeof(*new_alt_group));
 	if (!new_alt_group) {
···
 		return -1;
 	}
 
-	last_new_insn = NULL;
+	if (special_alt->new_len < special_alt->orig_len) {
+		/*
+		 * Insert a fake nop at the end to make the replacement
+		 * alt_group the same size as the original.  This is needed to
+		 * allow propagate_alt_cfi() to do its magic.  When the last
+		 * instruction affects the stack, the instruction after it (the
+		 * nop) will propagate the new state to the shared CFI array.
+		 */
+		nop = malloc(sizeof(*nop));
+		if (!nop) {
+			WARN("malloc failed");
+			return -1;
+		}
+		memset(nop, 0, sizeof(*nop));
+		INIT_LIST_HEAD(&nop->alts);
+		INIT_LIST_HEAD(&nop->stack_ops);
+		init_cfi_state(&nop->cfi);
+
+		nop->sec = special_alt->new_sec;
+		nop->offset = special_alt->new_off + special_alt->new_len;
+		nop->len = special_alt->orig_len - special_alt->new_len;
+		nop->type = INSN_NOP;
+		nop->func = orig_insn->func;
+		nop->alt_group = new_alt_group;
+		nop->ignore = orig_insn->ignore_alts;
+	}
+
+	if (!special_alt->new_len) {
+		*new_insn = nop;
+		goto end;
+	}
+
 	insn = *new_insn;
 	sec_for_each_insn_from(file, insn) {
 		struct reloc *alt_reloc;
···
 			continue;
 
 		dest_off = arch_jump_destination(insn);
-		if (dest_off == special_alt->new_off + special_alt->new_len) {
-			if (!fake_jump) {
-				WARN("%s: alternative jump to end of section",
-				     special_alt->orig_sec->name);
-				return -1;
-			}
-			insn->jump_dest = fake_jump;
-		}
+		if (dest_off == special_alt->new_off + special_alt->new_len)
+			insn->jump_dest = next_insn_same_sec(file, last_orig_insn);
 
 		if (!insn->jump_dest) {
 			WARN_FUNC("can't find alternative jump destination",
···
 		return -1;
 	}
 
+	if (nop)
+		list_add(&nop->list, &last_new_insn->list);
+end:
 	new_alt_group->orig_group = orig_alt_group;
 	new_alt_group->first_insn = *new_insn;
-	new_alt_group->last_insn = last_new_insn;
-
-	if (fake_jump)
-		list_add(&fake_jump->list, &last_new_insn->list);
-
+	new_alt_group->last_insn = nop ? : last_new_insn;
+	new_alt_group->cfi = orig_alt_group->cfi;
 	return 0;
 }
 
···
 	return 0;
 }
 
+/*
+ * The stack layouts of alternatives instructions can sometimes diverge when
+ * they have stack modifications.  That's fine as long as the potential stack
+ * layouts don't conflict at any given potential instruction boundary.
+ *
+ * Flatten the CFIs of the different alternative code streams (both original
+ * and replacement) into a single shared CFI array which can be used to detect
+ * conflicts and nicely feed a linear array of ORC entries to the unwinder.
+ */
+static int propagate_alt_cfi(struct objtool_file *file, struct instruction *insn)
+{
+	struct cfi_state **alt_cfi;
+	int group_off;
+
+	if (!insn->alt_group)
+		return 0;
+
+	alt_cfi = insn->alt_group->cfi;
+	group_off = insn->offset - insn->alt_group->first_insn->offset;
+
+	if (!alt_cfi[group_off]) {
+		alt_cfi[group_off] = &insn->cfi;
+	} else {
+		if (memcmp(alt_cfi[group_off], &insn->cfi, sizeof(struct cfi_state))) {
+			WARN_FUNC("stack layout conflict in alternatives",
+				  insn->sec, insn->offset);
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
 static int handle_insn_ops(struct instruction *insn, struct insn_state *state)
 {
 	struct stack_op *op;
 
 	list_for_each_entry(op, &insn->stack_ops, list) {
-		struct cfi_state old_cfi = state->cfi;
-		int res;
 
-		res = update_cfi_state(insn, &state->cfi, op);
-		if (res)
-			return res;
-
-		if (insn->alt_group && memcmp(&state->cfi, &old_cfi, sizeof(struct cfi_state))) {
-			WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
-			return -1;
-		}
+		if (update_cfi_state(insn, &state->cfi, op))
+			return 1;
 
 		if (op->dest.type == OP_DEST_PUSHF) {
 			if (!state->uaccess_stack) {
···
 	return 0;
 }
 
-/*
- * Alternatives should not contain any ORC entries, this in turn means they
- * should not contain any CFI ops, which implies all instructions should have
- * the same same CFI state.
- *
- * It is possible to constuct alternatives that have unreachable holes that go
- * unreported (because they're NOPs), such holes would result in CFI_UNDEFINED
- * states which then results in ORC entries, which we just said we didn't want.
- *
- * Avoid them by copying the CFI entry of the first instruction into the whole
- * alternative.
- */
-static void fill_alternative_cfi(struct objtool_file *file, struct instruction *insn)
+static struct instruction *next_insn_to_validate(struct objtool_file *file,
+						 struct instruction *insn)
 {
-	struct instruction *first_insn = insn;
 	struct alt_group *alt_group = insn->alt_group;
 
-	sec_for_each_insn_continue(file, insn) {
-		if (insn->alt_group != alt_group)
-			break;
-		insn->cfi = first_insn->cfi;
-	}
+	/*
+	 * Simulate the fact that alternatives are patched in-place.  When the
+	 * end of a replacement alt_group is reached, redirect objtool flow to
+	 * the end of the original alt_group.
+	 */
+	if (alt_group && insn == alt_group->last_insn && alt_group->orig_group)
+		return next_insn_same_sec(file, alt_group->orig_group->last_insn);
+
+	return next_insn_same_sec(file, insn);
 }
 
 /*
···
 	sec = insn->sec;
 
 	while (1) {
-		next_insn = next_insn_same_sec(file, insn);
+		next_insn = next_insn_to_validate(file, insn);
 
 		if (file->c_file && func && insn->func && func != insn->func->pfunc) {
 			WARN("%s() falls through to next function %s()",
···
 
 		insn->visited |= visited;
 
+		if (propagate_alt_cfi(file, insn))
+			return 1;
+
 		if (!insn->ignore_alts && !list_empty(&insn->alts)) {
 			bool skip_orig = false;
 
···
 					return ret;
 				}
 			}
-
-			if (insn->alt_group)
-				fill_alternative_cfi(file, insn);
 
 			if (skip_orig)
 				return 0;
···
 	if (!strcmp(insn->sec->name, ".fixup") ||
 	    !strcmp(insn->sec->name, ".altinstr_replacement") ||
 	    !strcmp(insn->sec->name, ".altinstr_aux"))
-		return true;
-
-	if (insn->type == INSN_JUMP_UNCONDITIONAL && insn->offset == FAKE_JUMP_OFFSET)
 		return true;
 
 	if (!insn->func)

tools/objtool/include/objtool/check.h

···
 
 	/* First and last instructions in the group */
 	struct instruction *first_insn, *last_insn;
+
+	/*
+	 * Byte-offset-addressed len-sized array of pointers to CFI structs.
+	 * This is shared with the other alt_groups in the same alternative.
+	 */
+	struct cfi_state **cfi;
 };
 
 struct instruction {

tools/objtool/orc_gen.c

···
 	return 0;
 }
 
+static unsigned long alt_group_len(struct alt_group *alt_group)
+{
+	return alt_group->last_insn->offset +
+	       alt_group->last_insn->len -
+	       alt_group->first_insn->offset;
+}
+
 int orc_create(struct objtool_file *file)
 {
 	struct section *sec, *ip_rsec, *orc_sec;
···
 			continue;
 
 		sec_for_each_insn(file, sec, insn) {
-			if (init_orc_entry(&orc, &insn->cfi))
-				return -1;
-			if (!memcmp(&prev_orc, &orc, sizeof(orc)))
+			struct alt_group *alt_group = insn->alt_group;
+			int i;
+
+			if (!alt_group) {
+				if (init_orc_entry(&orc, &insn->cfi))
+					return -1;
+				if (!memcmp(&prev_orc, &orc, sizeof(orc)))
+					continue;
+				if (orc_list_add(&orc_list, &orc, sec,
+						 insn->offset))
+					return -1;
+				nr++;
+				prev_orc = orc;
+				empty = false;
 				continue;
-			if (orc_list_add(&orc_list, &orc, sec, insn->offset))
-				return -1;
-			nr++;
-			prev_orc = orc;
-			empty = false;
+			}
+
+			/*
+			 * Alternatives can have different stack layout
+			 * possibilities (but they shouldn't conflict).
+			 * Instead of traversing the instructions, use the
+			 * alt_group's flattened byte-offset-addressed CFI
+			 * array.
+			 */
+			for (i = 0; i < alt_group_len(alt_group); i++) {
+				struct cfi_state *cfi = alt_group->cfi[i];
+				if (!cfi)
+					continue;
+				if (init_orc_entry(&orc, cfi))
+					return -1;
+				if (!memcmp(&prev_orc, &orc, sizeof(orc)))
+					continue;
+				if (orc_list_add(&orc_list, &orc, insn->sec,
+						 insn->offset + i))
+					return -1;
+				nr++;
+				prev_orc = orc;
+				empty = false;
+			}
+
+			/* Skip to the end of the alt_group */
+			insn = alt_group->last_insn;
 		}
 
 		/* Add a section terminator */