Build error with -fsanitize=address,undefined -O0

[guidovranken]

Set env:

export CC=clang-11
export CFLAGS="-fsanitize=address,undefined -O0"

(This is a valid combination of compile flags that works with any other project)

Then ./configure and build:

  CC       src/libsecp256k1_la-secp256k1.lo
In file included from src/secp256k1.c:13:
In file included from ./src/field_impl.h:18:
In file included from ./src/field_5x52_impl.h:18:
./src/field_5x52_asm_impl.h:29:5: error: inline assembly requires more registers than available
    "movq 0(%%rsi),%%r10\n"
    ^
./src/field_5x52_asm_impl.h:29:5: error: inline assembly requires more registers than available
./src/field_5x52_asm_impl.h:29:5: error: inline assembly requires more registers than available
./src/field_5x52_asm_impl.h:299:5: error: inline assembly requires more registers than available
    "movq 0(%%rsi),%%r10\n"
    ^
./src/field_5x52_asm_impl.h:299:5: error: inline assembly requires more registers than available
./src/field_5x52_asm_impl.h:299:5: error: inline assembly requires more registers than available
In file included from src/secp256k1.c:14:
In file included from ./src/scalar_impl.h:24:
./src/scalar_4x64_impl.h:279:5: error: inline assembly requires more registers than available
    "movq 32(%%rsi), %%r11\n"
    ^
./src/scalar_4x64_impl.h:279:5: error: inline assembly requires more registers than available
./src/scalar_4x64_impl.h:279:5: error: inline assembly requires more registers than available
9 errors generated.
Makefile:1054: recipe for target 'src/libsecp256k1_la-secp256k1.lo' failed

Tested with Clang 11 and latest repository checkout on Linux 64 bit.

[gmaxwell]

Looks like a clang bug to me. If it needs more registers for its own usage, its free to save something onto the stack to get enough for the assembly. Looks like there a number of unresolved reports of similar in their bug tracker: https://bugs.llvm.org/buglist.cgi?quicksearch=%22inline%20assembly%20requires%20more%20registers%20than%20available%22%20

You can disable assembly in this library or presumably use different compiler flags to avoid the issue.

[guidovranken]

Got it, thanks.

[sipa]

I remember when writing this code originally it was hard to not run out of registers, and initially it needed -fomit-frame-pointer. That's no longer needed, but we're probably still very close to the limit. If enabling these sanitizers reserves more registers from something else, it's not unreasonable that it causes failure.

[real-or-random]

Fwiw, you see essentially the same error with ./configure CFLAGS="-fsanitize=address -O0" CC=gcc But in the case of gcc, -fomit-frame-pointer is enough (which is in -O1). For clang, you apparently need some other flags in -O1. Just passing -O1 already works.

Can I ask why you compile with -O0? Is there some evidence that this provides "better" results for the sanitizers?

see #846 (comment) where I ran into the same issue.

[guidovranken]

-O0 can certainly bring to light bugs which do not manifest in high(er) optimization levels. A simple example is the statement

(void)strlen(s);

There's a good chance that will be optimized away with -O3 because it's essentially a no-op (the result of strlen isn't used, nor does strlen alter s, so removing it results in an equivalent program). But there's an OOB read there if s is an invalid pointer (or a string that is not NULL-terminated). Hence you can find this bug with -O0 but not -O3.

Similarly other code may be elided too when using optimization (according to any logic the compiler deems appropriate) so I occasionally run things with -O0 to get the most "verbatim" binary representation of the source code to potentially find new bugs.

Aggressive optimizations like -O3 can sometimes be useful to find undefined behavior (that's not detected by UBSAN, which only detects a small subset of all undefined behavior in C). If you start running into logic bugs or crashes that are not logical in relation to what the source code prescribes with -O3 that's often a manifestation of UB.

OSS-Fuzz strikes a balance by compiling everything with -O1 by default.