libsframe: fix sframe_find_fre for pltN entries

For a toy application on x86_64, for example, following is the SFrame
stack trace information for the 3 pltN entries of 16 bytes each:

   func idx [1]: pc = 0x401030, size = 48 bytes
   STARTPC[m]      CFA       FP        RA
   0000000000000000  sp+8      u         u
   000000000000000b  sp+16     u         u

The data in first column is the start_ip_offset.  Also note that the FDE
is of type SFRAME_FDE_TYPE_PCMASK (denoted by the [m] on LHS).

Where each pltN (note: excluding plt0 entry) entry looks like:

  401030: jmp    *0x2fca(%rip)
  401036: push   $0x0
  40103b: jmp    401020<_init+0x20>

  401040: jmp    *0x2fc2(%rip)
  401046: push   $0x1
  40104b: jmp    401020<_init+0x20>

  401050: jmp    *0x2fba(%rip)
  401056: push   $0x2
  40105b: jmp    401020<_init+0x20>

Now, to find SFrame stack trace information from an FDE of type
SFRAME_FDE_TYPE_PCMASK, sframe_find_fre () was doing an operation
like,
  (start_ip_offset & 0xf) >= (pc & 0xf)

This works for pltN entry of size, say, less than 16 bytes.  But if the
pltN entries or similar code stubs (for which SFrame FDE of type
SFRAME_FDE_TYPE_PCMASK may be used), evolve to be of size > 16 bytes,
this will cease to work.

To match the range covered by the SFrame FRE, one should instead perform
a modulo operation.  The constant for the modulo operation must be the
size of the pltN entry.  Further, this constant should ideally be
encoded in the format, as it may be different for each ABI.

In SFrame Version 2 of the format, we will move towards encoding it
explicitly in the SFrame FDE.  For now, fix up the logic to at least
move towards modulo operation.

libsframe/
	* sframe.c (sframe_fre_check_range_p): New definition.
	(sframe_find_fre): Refactor a bit and use the new definition
	above.
include/
	* sframe.h (SFRAME_FDE_TYPE_PCMASK): Update comment.
libsframe/doc/
	* sframe-spec.texi: Fix the text for SFRAME_FDE_TYPE_PCMASK FDE
	type.

2 files changed, 60 insertions, 24 deletions

diff --git a/libsframe/doc/sframe-spec.texi b/libsframe/doc/sframe-spec.texi
index 5155410..a37a6f9 100644
--- a/libsframe/doc/sframe-spec.texi
+++ b/libsframe/doc/sframe-spec.texi
@@ -451,8 +451,10 @@ entries and trampolines.
 
 @item SFRAME_FDE_TYPE_PCMASK
 @tab 1 @tab  Unwinders perform a @*
-(PC & FRE_START_ADDR_AS_MASK >= FRE_START_ADDR_AS_MASK)
-to look up a matching FRE.
+(PC % REP_BLOCK_SIZE @*
+ >= FRE_START_ADDR)
+to look up a matching FRE.  REP_BLOCK_SIZE is the size in bytes of the
+repeating block of program instructions.
 
 @end multitable
 
diff --git a/libsframe/sframe.c b/libsframe/sframe.c
index 2914243..fd966cf 100644
--- a/libsframe/sframe.c
+++ b/libsframe/sframe.c
@@ -362,6 +362,53 @@ sframe_decoder_get_funcdesc_at_index (sframe_decoder_ctx *ctx,
   return fdep;
 }
 
+/* Check whether for the given FDEP, the SFrame Frame Row Entry identified via
+   the START_IP_OFFSET and the END_IP_OFFSET, provides the stack trace
+   information for the PC.  */
+
+static bool
+sframe_fre_check_range_p (sframe_func_desc_entry *fdep,
+			  int32_t start_ip_offset, int32_t end_ip_offset,
+			  int32_t pc)
+{
+  int32_t start_ip, end_ip;
+  int32_t func_start_addr;
+  uint32_t rep_block_size;
+  uint32_t fde_type;
+  int32_t masked_pc;
+  bool mask_p;
+  bool ret;
+
+  ret = false;
+  /* FIXME - the rep_block_size should be encoded in the format somehow.  For
+     AMD64, each pltN entry stub in .plt is 16 bytes.  */
+  rep_block_size = 16;
+
+  if (!fdep)
+    return ret;
+
+  func_start_addr = fdep->sfde_func_start_address;
+  fde_type = sframe_get_fde_type (fdep);
+  mask_p = (fde_type == SFRAME_FDE_TYPE_PCMASK);
+
+  if (!mask_p)
+    {
+      start_ip = start_ip_offset + func_start_addr;
+      end_ip = end_ip_offset + func_start_addr;
+      ret = ((start_ip <= pc) && (end_ip >= pc));
+    }
+  else
+    {
+      /* For FDEs for repetitive pattern of insns, we need to return the FRE
+	 where pc % rep_block_size is between start_ip_offset and
+	 end_ip_offset.  */
+      masked_pc = pc % rep_block_size;
+      ret = ((start_ip_offset <= masked_pc) && (end_ip_offset >= masked_pc));
+    }
+
+  return ret;
+}
+
 static int
 flip_fre (char *fp, uint32_t fre_type, size_t *fre_size)
 {
@@ -1056,19 +1103,14 @@ 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, fde_type;
-  uint32_t end_ip_offset, i;
-  int32_t start_ip, end_ip;
+  uint32_t fre_type, fde_type, i;
+  int32_t start_ip_offset;
   int32_t func_start_addr;
+  int32_t end_ip_offset;
   const char *fres;
   size_t size = 0;
   int err = 0;
-  /* For regular FDEs (i.e. fde_type SFRAME_FDE_TYPE_PCINC),
-     where the start address in the FRE is an offset from start pc,
-     use a bitmask with all bits set so that none of the address bits are
-     ignored.  In this case, we need to return the FRE where
-     (PC >= FRE_START_ADDR) */
-  uint64_t bitmask = 0xffffffff;
+  bool mask_p;
 
   if ((ctx == NULL) || (frep == NULL))
     return sframe_set_errno (&err, SFRAME_ERR_INVAL);
@@ -1080,14 +1122,7 @@ sframe_find_fre (sframe_decoder_ctx *ctx, int32_t pc,
 
   fre_type = sframe_get_fre_type (fdep);
   fde_type = sframe_get_fde_type (fdep);
-
-  /* For FDEs for repetitive pattern of insns, we need to return the FRE
-     such that (PC & FRE_START_ADDR_AS_MASK >= FRE_START_ADDR_AS_MASK).
-     so, update the bitmask to the start address.  */
-  /* FIXME - the bitmask should be picked per ABI or encoded in the format
-     somehow. For AMD64, the pltN entry stub is 16 bytes. */
-  if (fde_type == SFRAME_FDE_TYPE_PCMASK)
-    bitmask = 0xf;
+  mask_p = (fde_type == SFRAME_FDE_TYPE_PCMASK);
 
   fres = ctx->sfd_fres + fdep->sfde_func_start_fre_off;
   func_start_addr = fdep->sfde_func_start_address;
@@ -1098,15 +1133,14 @@ sframe_find_fre (sframe_decoder_ctx *ctx, int32_t pc,
      if (err)
        return sframe_set_errno (&err, SFRAME_ERR_FRE_INVAL);
 
-     start_ip = func_start_addr + cur_fre.fre_start_addr;
+     start_ip_offset = cur_fre.fre_start_addr;
      end_ip_offset = sframe_fre_get_end_ip_offset (fdep, i, fres + size);
-     end_ip = func_start_addr + end_ip_offset;
 
-     if ((start_ip & bitmask) > (pc & bitmask))
+     /* First FRE's start_ip must be more than pc for regular SFrame FDEs.  */
+     if (i == 0 && !mask_p && (start_ip_offset + func_start_addr) > pc)
        return sframe_set_errno (&err, SFRAME_ERR_FRE_INVAL);
 
-     if (((start_ip & bitmask) <= (pc & bitmask))
-	 && (end_ip & bitmask) >= (pc & bitmask))
+     if (sframe_fre_check_range_p (fdep, start_ip_offset, end_ip_offset, pc))
        {
 	 sframe_frame_row_entry_copy (frep, &cur_fre);
 	 return 0;