Building chromium with clang coverage

[Manas Ghandat]

I am trying to build a coveage build for chromium and run into the following issue:

```
../../third_party/libFuzzer/src/FuzzerInterceptors.cpp:151:25: error: conflicting types for 'bcmp'
  151 | ATTRIBUTE_INTERFACE int bcmp(const char *s1, const char *s2, size_t n) {
      |                         ^
gen/build/modules/linux/../../../../../../build/linux/debian_bullseye_amd64-sysroot/usr/include/strings.h:34:12: note: previous
      declaration is here
   34 | extern int bcmp (const void *__s1, const void *__s2, size_t __n)
      |            ^
../../third_party/libFuzzer/src/FuzzerInterceptors.cpp:197:27: error: functions that differ only in their return type cannot be
      overloaded
  197 | ATTRIBUTE_INTERFACE char *strstr(const char *s1, const char *s2) {
      |                     ~~~~~~^
../../build/linux/debian_bullseye_amd64-sysroot/usr/include/string.h:323:1: note: previous definition is here
  322 | __extern_always_inline const char *
      |                              ~~~~~~
  323 | strstr (const char *__haystack, const char *__needle) __THROW
      | ^
../../third_party/libFuzzer/src/FuzzerInterceptors.cpp:205:27: error: functions that differ only in their return type cannot be
      overloaded
  205 | ATTRIBUTE_INTERFACE char *strcasestr(const char *s1, const char *s2) {
      |                     ~~~~~~^
../../build/linux/debian_bullseye_amd64-sysroot/usr/include/string.h:356:26: note: previous declaration is here
  356 | extern "C++" const char *strcasestr (const char *__haystack,
```

Event after fixing the `FuzzerInterceptors.cpp` file, there are stil errors that persist. Below is my compilation approach:

```
gn gen out/coverage --args='''
dcheck_always_on = false

is_debug = false
use_clang_coverage = true
is_component_build = false
optimize_for_fuzzing = false
use_remoteexec = false
is_asan = false
use_libfuzzer = true
symbol_level = 2'''

autoninja -C out/coverage
```

If anyone has a fix, do let me know

[Giovanni Ortuño]

Hi,

How come you're using `use_libfuzzer = true`? Are you trying to compile a fuzzer?

We don't support running fuzzers with code coverage enabled.

Gio

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[Manas Ghandat]

Hi,

My goal is generating coverage for the input corpus that I have got after fuzzing. That would require me to generate a fuzz target (thus `use_libfuzzer=true`) and use clang coverage. I was following the instructions mentioned here: https://github.com/chromium/chromium/blob/main/tools/code_coverage/coverage.py#L38

Thanks,
Manas

On 3/6/26 20:34, Giovanni Ortuño wrote:
> Hi,
>
> How come you're using `use_libfuzzer = true`? Are you trying to compile a fuzzer?
>
> We don't support running fuzzers with code coverage enabled.
>
> Gio
>

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[Giovanni Ortuño]

Ah, I was mistaken. We do support building fuzzers with code coverage. Looking at the code coverage bot, I see this gn argument that you might be missing:

```
use_clang_modules = false
```

Mind trying that?

[Manas Ghandat]

Thank you so much, the suggested method worked.

On 3/6/26 21:11, Giovanni Ortuño wrote:
> Ah, I was mistaken. We do support building fuzzers with code coverage. Looking at the code coverage bot, I see this gn argument that you might be missing:
>
> ```
> use_clang_modules = false
> ```
>
> Mind trying that?
>

> On Fri, Mar 6, 2026 at 10:24 AM Manas Ghandat <ghanda...@gmail.com <mailto:ghanda...@gmail.com>> wrote:
>
> Hi,
>

> My goal is generating coverage for the input corpus that I have got after fuzzing. That would require me to generate a fuzz target (thus `use_libfuzzer=true`) and use clang coverage. I was following the instructions mentioned here: https://github.com/chromium/chromium/blob/main/tools/code_coverage/coverage.py#L38 <https://github.com/chromium/chromium/blob/main/tools/code_coverage/coverage.py#L38>

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>

[Nico Weber]

(Modules not working in coverage builds is crbug.com/440481921)

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[Junji Watanabe]

It sounds like we need to disable use_clang_modules when use_clang_coverage is enabled. 

We will follow up on it.

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/chromium-dev/CADZ1Xia%2Br7%2BAaAMq-QFb0w4o4W8HhThkcPiS6Poz7DtFkYPzgQ%40mail.gmail.com.

[Sidhin Thomas]

Apologies, wrong thread.

On Wed, Mar 11, 2026 at 4:30 PM Sidhin Thomas <sidhin...@gmail.com> wrote:

Inspired by the thread. I tried another hand on RBE and got native link working. The benefits from caching are huge vs ccache. But in order to get it working I had to hard code the container-image from google.star while building the worker image. I'll update it every now and then.

Any idea, how much and what kind of issues can I expect due to toolchain drift when using the RBE for dev workflows? C++ in general is ABI compatible so I am assuming it should be safe. 
(I am basically ignoring toolchain as a match condition for workers)

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/chromium-dev/CAPi%2BNeXKae0%2Bfhy0SfSLRGOZwzpBFVJX%2B%3DDkdVezryfvMY%3D5QA%40mail.gmail.com.

[Sidhin Thomas]

Inspired by the thread. I tried another hand on RBE and got native link working. The benefits from caching are huge vs ccache. But in order to get it working I had to hard code the container-image from google.star while building the worker image. I'll update it every now and then.

Any idea, how much and what kind of issues can I expect due to toolchain drift when using the RBE for dev workflows? C++ in general is ABI compatible so I am assuming it should be safe. 
(I am basically ignoring toolchain as a match condition for workers)

On Tue, Mar 10, 2026 at 7:38 AM 'Junji Watanabe' via Chromium-dev <chromi...@chromium.org> wrote:

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/chromium-dev/CAPi%2BNeXKae0%2Bfhy0SfSLRGOZwzpBFVJX%2B%3DDkdVezryfvMY%3D5QA%40mail.gmail.com.