From 59ec363d7236d92f896135ef8fe85433d6f1995e Mon Sep 17 00:00:00 2001

From: Serhei Makarov <smakarov@redhat.com>

Date: Fri, 9 Nov 2018 16:24:09 -0500

Subject: [PATCH 27/32] PR23860: reduce stack pressure from format strings

Reduce stack pressure created by the earlier commits by allocating

format strings in a predictable location in the top half of the stack

[-BPF_MAXSTRINGLEN*2..0) as long as they fit in there. This works

since only one format string is active at a time and no ordinary

strings are being allocated in that region of the stack now.

* bpf-opt.cxx (alloc_literal_str): Store format_str in top half.

---

 bpf-opt.cxx | 11 +++++++++++

 1 file changed, 11 insertions(+)

diff --git a/bpf-opt.cxx b/bpf-opt.cxx

index f3aa5c462..5c7acb6b9 100644

--- a/bpf-opt.cxx

+++ b/bpf-opt.cxx

@@ -27,6 +27,16 @@ alloc_literal_str(program &p, insn_inserter &ins, value *s)

   str_bytes += 4 - str_bytes % 4; // write aligned words to avoid garbage data

   int ofs; size_t tmp_space;

+  if (s->is_format() && str_bytes <= BPF_MAXSTRINGLEN * 2)

+    {

+      // PR23068 workaround mitigation to reduce stack pressure:

+      //

+      // Store format strings in the top of the stack, since at most

+      // one printf() operation is prepared at a time and other string

+      // values will not be stored in that area now.

+      ofs = -str_bytes;

+      goto write_string;

+    }

   // Append the string to existing temporary data.

   //

@@ -65,6 +75,7 @@ alloc_literal_str(program &p, insn_inserter &ins, value *s)

   p.use_tmp_space(tmp_space);

   ofs = -tmp_space;

+ write_string:

   value *frame = p.lookup_reg(BPF_REG_10);

   value *out = emit_simple_literal_str(p, ins, frame, ofs, str, false /* don't zero pad */);

   return out;

-- 

2.14.5