libsframe: handle SFrame FRE start/end IP offsets as unsigned

The SFrame FRE start address (fre_start_addr) is defined as unsigned
32-bit integer, as it is an offset from SFrame FDE function start
address (sfde_func_start_address) and functions only grow upwards
(towards higher addresses).

The SFrame FRE start IP offset is a synonym to the SFrame FRE start
address.  The SFrame FRE end IP offset is either the value of the
subsequent FDE start address minus one, if that exists, or the FDE
function size minus one otherwise.  Both should therefore be handled
as unsigned 32-bit integer.

In libsframe the "lookup PC" (pc) and SFrame FDE function start address
(sfde_func_start_address) are both signed integers, as they are actually
offsets from the SFrame section.  The unsigned FDE start/end IP offsets
may therefore only be safely compared against the offset of the lookup
PC from FDE function start address if the FDE function start address is
lower or equal to the lookup PC, as this guarantees the offset to be
always positive:

Given:

  lookup_pc = pc - sframe_addr

  sfde_func_start_address = func_start_addr - sframe_addr

If the FDE function start address is lower or equal than the lookup PC,
which both are signed offsets from SFrame section, then the function
start address is also lower or equal to the PC, which are both unsigned:

  sfde_func_start_address <= lookup_pc
  func_start_addr - sframe_addr <= pc - sframe_addr
  func_start_addr <= pc

With that the offset of the lookup PC from FDE function start address
(lookup_pc - sfde_func_start_address) must always be positive, if
FDE function start address is lower or equal to the lookup PC:

  lookup_pc - sfde_func_start_address
  = pc - sframe_addr - (func_start_addr - sframe_addr)
  = pc - func_start_addr

libsframe/
	* sframe.c (sframe_find_fre): Define and handle start_ip_offset
	and end_ip_offset as unsigned (same as FRE fre_start_addr).
	(sframe_fre_check_range_p): Likewise.  Define PC offset (from
	function start address) as unsigned.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>

diff --git a/libsframe/sframe.c b/libsframe/sframe.c
index d03dd5fc5fed9ea5ec542a0fbb89b175e286b36e..950a6846f042d0e520761f3f1f6e35d5cb0efd84 100644 (file)
--- a/libsframe/sframe.c
+++ b/libsframe/sframe.c
@@ -369,13 +369,13 @@ sframe_decoder_get_funcdesc_at_index (sframe_decoder_ctx *ctx,
 
 static bool
 sframe_fre_check_range_p (sframe_func_desc_entry *fdep,
-                         int32_t start_ip_offset, int32_t end_ip_offset,
+                         uint32_t start_ip_offset, uint32_t end_ip_offset,
                          int32_t pc)
 {
   int32_t func_start_addr;
   uint8_t rep_block_size;
   uint32_t fde_type;
-  int32_t pc_offset;
+  uint32_t pc_offset;
   bool mask_p;
 
   if (!fdep)
@@ -386,6 +386,10 @@ sframe_fre_check_range_p (sframe_func_desc_entry *fdep,
   mask_p = (fde_type == SFRAME_FDE_TYPE_PCMASK);
   rep_block_size = fdep->sfde_func_rep_size;
 
+  if (func_start_addr > pc)
+    return false;
+
+  /* Given func_start_addr <= pc, pc - func_start_addr must be positive.  */
   pc_offset = pc - func_start_addr;
   /* For SFrame FDEs encoding information for repetitive pattern of insns,
      masking with the rep_block_size is necessary to find the matching FRE.  */
@@ -1097,9 +1101,8 @@ sframe_find_fre (sframe_decoder_ctx *ctx, int32_t pc,
   sframe_frame_row_entry cur_fre;
   sframe_func_desc_entry *fdep;
   uint32_t fre_type, i;
-  int32_t start_ip_offset;
   int32_t func_start_addr;
-  int32_t end_ip_offset;
+  uint32_t start_ip_offset, end_ip_offset;
   const char *fres;
   size_t size = 0;
   int err = 0;
@@ -1126,8 +1129,9 @@ sframe_find_fre (sframe_decoder_ctx *ctx, int32_t pc,
      start_ip_offset = cur_fre.fre_start_addr;
      end_ip_offset = sframe_fre_get_end_ip_offset (fdep, i, fres + size);
 
-     /* Stop search if FRE's start_ip is greater than pc.  */
-     if ((start_ip_offset + func_start_addr) > pc)
+     /* Stop search if FRE's start_ip is greater than pc.  Given
+       func_start_addr <= pc, pc - func_start_addr must be positive.  */
+     if (start_ip_offset > (uint32_t)(pc - func_start_addr))
        return sframe_set_errno (&err, SFRAME_ERR_FRE_INVAL);
 
      if (sframe_fre_check_range_p (fdep, start_ip_offset, end_ip_offset, pc))