runtime: handle bad ftab index in symtab.go

If a program has had its text section split into multiple
sections then the ftab that is built is based on addresses
prior to splitting.  That means all the function addresses
are there and correct because of relocation but the
but the computed idx won't always match up quite right and
in some cases go beyond the end of the table, causing a panic.

To resolve this, determine if the idx is too large and if it is,
set it to the last index in ftab.  Then search backward to find the
matching function address.

Fixes #17854

Change-Id: I6940e76a5238727b0a9ac23dc80000996db2579a
Reviewed-on: https://go-review.googlesource.com/32972Reviewed-by: David Chase <drchase@google.com>

src/runtime/symtab.go

@@ -442,11 +442,18 @@ func findfunc(pc uintptr) *_func {
 
 	ffb := (*findfuncbucket)(add(unsafe.Pointer(datap.findfunctab), b*unsafe.Sizeof(findfuncbucket{})))
 	idx := ffb.idx + uint32(ffb.subbuckets[i])
+
+	// If the idx is beyond the end of the ftab, set it to the end of the table and search backward.
+	// This situation can occur if multiple text sections are generated to handle large text sections
+	// and the linker has inserted jump tables between them.
+
+	if idx >= uint32(len(datap.ftab)) {
+		idx = uint32(len(datap.ftab) - 1)
+	}
 	if pc < datap.ftab[idx].entry {
 
-		// If there are multiple text sections then the buckets for the secondary
-		// text sections will be off because the addresses in those text sections
-		// were relocated to higher addresses.  Search back to find it.
+		// With multiple text sections, the idx might reference a function address that
+		// is higher than the pc being searched, so search backward until the matching address is found.
 
 		for datap.ftab[idx].entry > pc && idx > 0 {
 			idx--