For reference, the full output from msan looks like:

==4872==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0xaaaac681ef98 in tb_gen_code third_party/qemu/accel/tcg/translate-all.c:358:21
    #1 0xaaaac67f2520 in cpu_exec_loop third_party/qemu/accel/tcg/cpu-exec.c:993:22
    #2 0xaaaac67f154c in cpu_exec_setjmp third_party/qemu/accel/tcg/cpu-exec.c:1039:12
    #3 0xaaaac67f1240 in cpu_exec third_party/qemu/accel/tcg/cpu-exec.c:1065:11
    #4 0xaaaac681022c in tcg_cpu_exec third_party/qemu/accel/tcg/tcg-accel-ops.c:79:11
    #5 0xaaaac680ede4 in mttcg_cpu_thread_fn third_party/qemu/accel/tcg/tcg-accel-ops-mttcg.c:95:17
    #6 0xaaaacf096698 in qemu_thread_start third_party/qemu/util/qemu-thread-posix.c:541:9
    #7 0xffffa9242cec in start_thread (/usr/grte/v5/lib64/libpthread.so.0+0xbcec) (BuildId: 0bdac2117d4465a78d3de57b307368b4)
    #8 0xffffa912ec98 in thread_start (/usr/grte/v5/lib64/libc.so.6+0x116c98) (BuildId: 613d20d3b812b4c87fe9ebf8c4caae83)

  Uninitialized value was stored to memory at
    #0 0xaaaac681ef94 in tb_gen_code third_party/qemu/accel/tcg/translate-all.c:358:50
    #1 0xaaaac67f2520 in cpu_exec_loop third_party/qemu/accel/tcg/cpu-exec.c:993:22
    #2 0xaaaac67f154c in cpu_exec_setjmp third_party/qemu/accel/tcg/cpu-exec.c:1039:12
    #3 0xaaaac67f1240 in cpu_exec third_party/qemu/accel/tcg/cpu-exec.c:1065:11
    #4 0xaaaac681022c in tcg_cpu_exec third_party/qemu/accel/tcg/tcg-accel-ops.c:79:11
    #5 0xaaaac680ede4 in mttcg_cpu_thread_fn third_party/qemu/accel/tcg/tcg-accel-ops-mttcg.c:95:17
    #6 0xaaaacf096698 in qemu_thread_start third_party/qemu/util/qemu-thread-posix.c:541:9
    #7 0xffffa9242cec in start_thread (/usr/grte/v5/lib64/libpthread.so.0+0xbcec) (BuildId: 0bdac2117d4465a78d3de57b307368b4)
    #8 0xffffa912ec98 in thread_start (/usr/grte/v5/lib64/libc.so.6+0x116c98) (BuildId: 613d20d3b812b4c87fe9ebf8c4caae83)

  Uninitialized value was created by an allocation of 'host_pc' in the stack frame
    #0 0xaaaac681d8ac in tb_gen_code third_party/qemu/accel/tcg/translate-all.c:297:5

SUMMARY: MemorySanitizer: use-of-uninitialized-value third_party/qemu/accel/tcg/translate-all.c:358:21 in tb_gen_code
Exiting

On Fri, Feb 28, 2025 at 5:26 PM Patrick Venture <venture@google.com> wrote:

On Fri, Feb 28, 2025 at 1:38 PM Richard Henderson <richard.henderson@linaro.org> wrote:
On 2/28/25 13:23, Patrick Venture wrote:
> From: Peter Foley <pefoley@google.com>
>
> e.g.
> Uninitialized value was created by an allocation of 'host_pc' in the stack frame
> #0 0xaaaac07df87c in tb_gen_code third_party/qemu/accel/tcg/translate-all.c:297:5
>
> Signed-off-by: Peter Foley <pefoley@google.com>
> Signed-off-by: Patrick Venture <venture@google.com>
> ---
> accel/tcg/translate-all.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index d4189c7386..f584055a15 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -298,7 +298,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
> tcg_insn_unit *gen_code_buf;
> int gen_code_size, search_size, max_insns;
> int64_t ti;
> - void *host_pc;
> + void *host_pc = NULL;
>
> assert_memory_lock();
> qemu_thread_jit_write();

False positive, because the error return exits without using the uninitialized value.
But if we do want to "fix" this, do it at the beginning of get_page_addr_code_hostp.

Acknowledged. Gotta hate false positives, although better aggressive than not. Thanks!

r~