[PATCH 03/10] tools/insn-fuzz: Don't use memcpy() for zero-length reads

[Andrew Cooper <andrew.cooper3@citrix.com>]

For control-flow changes, the emulator needs to perform a zero-length
instruction fetch at the target offset.  It also passes NULL for the
destination buffer, as there is no instruction stream to collect.

This trips up UBSAN, even with a size of 0.  Exclude zero-length reads from
using memcpy(), rather than excluding NULL destination pointers, to still
catch unintentional uses of NULL.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
---
 tools/fuzz/x86_instruction_emulator/fuzz-emul.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
index 890642c..cbdb3dd 100644
--- a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
+++ b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
@@ -77,7 +77,7 @@ static int data_read(const char *why, void *dst, unsigned int bytes)
     else
         rc = maybe_fail(why, true);
 
-    if ( rc == X86EMUL_OKAY )
+    if ( rc == X86EMUL_OKAY && bytes )
     {
         memcpy(dst,  input.data + data_index, bytes);
         data_index += bytes;
-- 
2.1.4


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