Trampoline fixes.

The transformed code was incorrect because it assumed the pointer it was
writing to was where the code would execute, but it was actually
'rewritten_temp'.  Changed transform_dis_main to take a pc_trampoline
pointer, which also helps the test harness.  However, this means that it
has to be called after the trampoline has been allocated, while before
the trampoline allocation depended on the generated size; this change
doesn't bother to use two passes or anything, but just allocates a new
code buffer if the maximum possible size isn't available - not the end
of the world, since trampoline_ptr will still only be increased by the
actual size before the next hook in the series (if any).

1 files changed, 14 insertions, 7 deletions

diff --git a/lib/x86/arch-transform-dis.inc.h b/lib/x86/arch-transform-dis.inc.h
index 1e5d872..6ec6c16 100644
--- a/lib/x86/arch-transform-dis.inc.h
+++ b/lib/x86/arch-transform-dis.inc.h
@@ -23,8 +23,10 @@ static inline void push_mov_tail(void **code, bool rax) {
 UNUSED
 static void transform_dis_pcrel(struct transform_dis_ctx *ctx, uint64_t dpc,
                                 struct arch_pcrel_info info) {
-    /* push %reg; mov $dpc, %reg; <orig but with reg instead>; pop %reg */
-    /* reg is rcx, or rax if the instruction might be using rcx. */
+    /* push %reg; mov $dpc, %reg; <orig but with reg instead>; pop %reg
+     * reg is rcx, or rax if the instruction might be using rcx.
+     * Max size: 11 + orig + 1
+     * Minimum size is 6 bytes, so there could be at most 1 in a patch area. */
     bool rax = info.reg == 1;
     void *code = *ctx->rewritten_ptr_ptr;
     push_mov_head(&code, dpc, rax);
@@ -32,7 +34,7 @@ static void transform_dis_pcrel(struct transform_dis_ctx *ctx, uint64_t dpc,
     code += ctx->base.op_size;
     push_mov_tail(&code, rax);
     *ctx->rewritten_ptr_ptr = code;
-    ctx->base.newop[0] = rax ? 0 : 1;
+    ctx->base.newval[0] = rax ? 0 : 1;
     ctx->base.modify = true;
 }
 
@@ -41,7 +43,9 @@ static void transform_dis_branch(struct transform_dis_ctx *ctx, uint_tptr dpc,
     if (dpc >= ctx->pc_patch_start && dpc < ctx->pc_patch_end) {
         if (dpc == ctx->base.pc + ctx->base.op_size && (cc & CC_CALL)) {
             /* Probably a poor man's PC-rel - 'call .; pop %some'.
-             * Push the original address. */
+             * Push the original address.
+             * Max size: orig + 1 + 11 + 5 + 1
+             * Minimum call size is 4 bytes; at most 2. */
             void *code = *ctx->rewritten_ptr_ptr;
             ctx->write_newop_here = NULL;
 
@@ -72,15 +76,18 @@ static void transform_dis_branch(struct transform_dis_ctx *ctx, uint_tptr dpc,
     code += ctx->base.op_size;
 
     struct arch_dis_ctx arch;
-    uintptr_t source = (uintptr_t) code + 2;
+    uintptr_t source = ctx->pc_trampoline + 2;
     int size = jump_patch_size(source, dpc, arch, true);
-    /* if not taken, jmp past the big jump - this is a bit suboptimal but not that bad */
+    /* If not taken, jmp past the big jump - this is a bit suboptimal but not
+     * that bad.
+     * Max size: orig + 2 + 14
+     * Minimum jump size is 2 bytes; at most 3. */
     op8(&code, 0xeb);
     op8(&code, size);
     make_jump_patch(&code, source, dpc, arch);
 
     *ctx->rewritten_ptr_ptr = code;
-    ctx->base.newop[0] = 2;
+    ctx->base.newval[0] = 2;
     ctx->base.modify = true;
 
     if (!cc)