commit 36d001c70d8a0144ac1d038f6876c484849a74de upstream. On 64 bits, we always, by necessity, jump through the system call table via %rax. For 32-bit system calls, in theory the system call number is stored in %eax, and the code was testing %eax for a valid system call number. At one point we loaded the stored value back from the stack to enforce zero-extension, but that was removed in checkin d4d67150165df8bf1cc05e532f6efca96f907cab. An actual 32-bit process will not be able to introduce a non-zero-extended number, but it can happen via ptrace. Instead of re-introducing the zero-extension, test what we are actually going to use, i.e. %rax. This only adds a handful of REX prefixes to the code. Reported-by: Ben Hawkes <hawkes@sota.gen.nz> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com> Cc: Roland McGrath <roland@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>