UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0

Steps to reproduce the problem:
1. Attempt to build Chromium 59 on a Linux platform using glibc where <asm/unistd.h> does not define __NR_getrandom.

What is the expected behavior?
It compiles.

What went wrong?
[4787/27905] arm-linux-gnueabihf-gcc -MMD -MF obj/third_party/boringssl/boringssl/urandom.o.d -DV8_DEPRECATION_WARNINGS -DUSE_UDEV -DUSE_AURA=1 -DUSE_PANGO=1 -DUSE_CAIRO=1 -DUSE_GLIB=1 -DUSE_NSS_CERTS=1 -DUSE_X11=1 -DNO_TCMALLOC -DDISABLE_NACL -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1 -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -DCOMPONENT_BUILD -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DBORINGSSL_IMPLEMENTATION -DBORINGSSL_NO_STATIC_INITIALIZER -DOPENSSL_SMALL -D_XOPEN_SOURCE=700 -DBORINGSSL_SHARED_LIBRARY -I../.. -Igen -I../../third_party/boringssl/src/include -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -funwind-tables -fPIC -pipe -march=armv7-a -mfloat-abi=hard -mtune=generic-armv7-a -pthread -mfpu=neon -mthumb -O2 -fno-ident -fdata-sections -ffunction-sections -fomit-frame-pointer -g1 -fvisibility=hidden -Wno-psabi -Wno-unused-local-typedefs -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter -std=c99 -fno-delete-null-pointer-checks -c ../../third_party/boringssl/src/crypto/rand/urandom.c -o obj/third_party/boringssl/boringssl/urandom.o
FAILED: obj/third_party/boringssl/boringssl/urandom.o 
arm-linux-gnueabihf-gcc -MMD -MF obj/third_party/boringssl/boringssl/urandom.o.d -DV8_DEPRECATION_WARNINGS -DUSE_UDEV -DUSE_AURA=1 -DUSE_PANGO=1 -DUSE_CAIRO=1 -DUSE_GLIB=1 -DUSE_NSS_CERTS=1 -DUSE_X11=1 -DNO_TCMALLOC -DDISABLE_NACL -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1 -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -DCOMPONENT_BUILD -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DBORINGSSL_IMPLEMENTATION -DBORINGSSL_NO_STATIC_INITIALIZER -DOPENSSL_SMALL -D_XOPEN_SOURCE=700 -DBORINGSSL_SHARED_LIBRARY -I../.. -Igen -I../../third_party/boringssl/src/include -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -funwind-tables -fPIC -pipe -march=armv7-a -mfloat-abi=hard -mtune=generic-armv7-a -pthread -mfpu=neon -mthumb -O2 -fno-ident -fdata-sections -ffunction-sections -fomit-frame-pointer -g1 -fvisibility=hidden -Wno-psabi -Wno-unused-local-typedefs -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter -std=c99 -fno-delete-null-pointer-checks -c ../../third_party/boringssl/src/crypto/rand/urandom.c -o obj/third_party/boringssl/boringssl/urandom.o
../../third_party/boringssl/src/crypto/rand/urandom.c:62:2: error: #error "system call number for getrandom is not the expected value"
 #error "system call number for getrandom is not the expected value"
  ^
In file included from /usr/include/arm-linux-gnueabihf/sys/syscall.h:31:0,
                 from ../../third_party/boringssl/src/crypto/rand/urandom.c:32:
../../third_party/boringssl/src/crypto/rand/urandom.c: In function 'init_once':
../../third_party/boringssl/src/crypto/rand/urandom.c:128:15: error: '__NR_getrandom' undeclared (first use in this function)
       syscall(SYS_getrandom, &dummy, sizeof(dummy), GRND_NONBLOCK);
               ^
../../third_party/boringssl/src/crypto/rand/urandom.c:128:15: note: each undeclared identifier is reported only once for each function it appears in
../../third_party/boringssl/src/crypto/rand/urandom.c: In function 'fill_with_entropy':
../../third_party/boringssl/src/crypto/rand/urandom.c:258:21: error: '__NR_getrandom' undeclared (first use in this function)
         r = syscall(SYS_getrandom, out, len, 0 /* no flags */);
                     ^
ninja: build stopped: subcommand failed.

The reason is that glibc <sys/syscall.h> has an entry like `#define SYS_getrandom __NR_getrandom` and otherwise relies on the kernel's definitions of these __NR_foo values coming from asm/unistd.h. If this kernel asm/unistd.h is old, it will not define __NR_getrandom, so SYS_getrandom ends up defined, but not actually declared/usable. The check in urandom.c is whether, on Linux, SYS_getrandom is defined - which it is - so it enters the path of comparing SYS_getrandom with EXPECTED_SYS_getrandom and fails with #error.

It looks to me that in the case of Linux and glibc, this macro trickery of having a fallback value for SYS_getrandom will not do it's intended job if glibc's syscall.h is newer than the kernel's unistd.h. I fixed it in our build (at least until we get new kernel headers!) by adding a simple check:
#if !defined(__NR_getrandom)
#undef SYS_getrandom
#endif

That's definitely not the "right" fix, but to me all of this macro checking seems kind of weird and may just be a legacy from a time when the syscall was newly-added / not stably numbered. Given that the syscall is runtime-detected anyway, and the current macro checks means it won't use the syscall unless an EXPECTED_SYS_getrandom is already included in the file for the architecture... you already have all of the syscall numbers anyway, and cross-checking with the C library introduces scope for spurious errors. You could just do something like this:

#ifdef EXPECTED_SYS_getrandom
#undef SYS_getrandom
#define SYS_getrandom EXPECTED_SYS_getrandom
#endif

#ifdef SYS_getrandom
#define USE_SYS_getrandom
#endif

Then it will use your hardcoded numbers on "known" architectures, or the C library provided number otherwise, and not #error unnecessarily.

Other parts of Chromium use things like sandbox/linux/system_headers/arm_linux_syscalls.h to just ensure that the syscall numbers are always defined, and hence break the dependency on asm/unistd.h being complete. Also a solid option - cut out the need to maintain a syscall ABI table inside your library and you can just delete all of that macro stuff.

Did this work before? No 

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