1 // Copyright 2018 The Chromium Authors 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #include "base/posix/can_lower_nice_to.h" 6 7 #include <limits.h> 8 #include <sys/resource.h> 9 #include <sys/types.h> 10 #include <unistd.h> 11 12 #include "build/build_config.h" 13 14 // Not defined on AIX by default. 15 #if BUILDFLAG(IS_AIX) 16 #if defined(RLIMIT_NICE) 17 #error Assumption about OS_AIX is incorrect 18 #endif 19 #define RLIMIT_NICE 20 20 #endif 21 22 namespace base { 23 namespace internal { 24 CanLowerNiceTo(int nice_value)25bool CanLowerNiceTo(int nice_value) { 26 // On a POSIX system, the nice value of a thread can be lowered 1. by the root 27 // user, 2. by a user with the CAP_SYS_NICE permission or 3. by any user if 28 // the target value is within the range allowed by RLIMIT_NICE. 29 30 // 1. Check for root user. 31 if (geteuid() == 0) 32 return true; 33 34 // 2. Skip checking the CAP_SYS_NICE permission because it would require 35 // libcap.so. 36 37 // 3. Check whether the target value is within the range allowed by 38 // RLIMIT_NICE. 39 // 40 // NZERO should be defined in <limits.h> per POSIX, and should be at least 20. 41 // (NZERO-1) is the highest possible niceness value (i.e. lowest priority). 42 // Most platforms use NZERO=20. 43 // 44 // RLIMIT_NICE tells us how much we can reduce niceness (increase priority) if 45 // we start at NZERO. For example, if NZERO is 20 and the rlimit is 30, we can 46 // lower niceness anywhere within the [-10, 19] range (20 - 30 = -10). 47 // 48 // So, we are allowed to reduce niceness to a minimum of NZERO - rlimit: 49 struct rlimit rlim; 50 if (getrlimit(RLIMIT_NICE, &rlim) != 0) 51 return false; 52 const int lowest_nice_allowed = NZERO - static_cast<int>(rlim.rlim_cur); 53 54 // And lowering niceness to |nice_value| is allowed if it is greater than or 55 // equal to the limit: 56 return nice_value >= lowest_nice_allowed; 57 } 58 59 } // namespace internal 60 } // namespace base 61