[fuchsia] Add loop to _zx_cprng_draw.

[flowerhack]

Summary

The _zx_cprng_draw API expects to be looped rather than simply
called. Update the code to reflect this.

[mcgrathr]

LGTM

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

494

Pointer arithmetic on void* is a GNU extension, but fine if it's allowed in this code.

[cryptoad]

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

490

length being a uptr, the condition comes down to while length != 0. I assume size is guaranteed to not be > length?

[flowerhack]

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

490

that's correct; size will never be > length

494

interesting! how would i know whether a particular extension is allowed in a part of compiler-rt? i couldn't find anything about it in the LLVM coding standards

[mcgrathr]

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

494

If it compiles then they haven't chosen -std=... options to enforce it, which means either they think it's fine or they don't really care.
As to the intent, the guys on the team will have to speak up.

[flowerhack]

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

494

sgtm

[flowerhack]

added a couple reviewers with commit bits

please feel free to hit the "ship it" button on my behalf or, if that should not happen, let me know what to fix!

[phosek]

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

494

Make sure you try to compile this code with Clang as your host compiler.

[phosek]

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

494

I tried build our toolchain with this patch (which uses Clang as a first stage compiler) and I'm getting an error on this line: error: arithmetic on a pointer to void

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cc

 uptr ReadLongProcessName(/*out*/ char *buf, uptr buf_len) {
   return ReadBinaryName(buf, buf_len);
 }
 
 uptr MainThreadStackBase, MainThreadStackSize;
 
 bool GetRandom(void *buffer, uptr length, bool blocking) {
   CHECK_LE(length, ZX_CPRNG_DRAW_MAX_LEN);
+  
+  while (length > 0) {
     size_t size;
-  CHECK_EQ(_zx_cprng_draw(buffer, length, &size), ZX_OK);
-  CHECK_EQ(size, length);
+    _zx_cprng_draw(buffer, length, &size);
+    CHECK_EQ(status, ZX_OK);
+    buffer += size;
+    length -= size;
+  }
   return true;
 }
 
 }  // namespace __sanitizer
 
 using namespace __sanitizer;  // NOLINT
 
 extern "C" {