[Valgrind-developers] [valgrind] Explain why realloc_size_zero does not use Addr.Block and clean up a volatile cast

[Paul F. <pa...@so...>]

https://sourceware.org/cgit/valgrind/commit/?id=b6903333fea9e3fdce97e6508bcf59a448b9ef52

commit b6903333fea9e3fdce97e6508bcf59a448b9ef52
Author: Paul Floyd <pj...@wa...>
Date:   Sun Mar 1 18:19:46 2026 +0100

    Explain why realloc_size_zero does not use Addr.Block and clean up a volatile cast

Diff:
---
 coregrind/m_syswrap/syswrap-freebsd.c | 10 +---------
 memcheck/mc_errors.c                  |  9 +++++++++
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/coregrind/m_syswrap/syswrap-freebsd.c b/coregrind/m_syswrap/syswrap-freebsd.c
index c2e2f50dc9..c9a3939478 100644
--- a/coregrind/m_syswrap/syswrap-freebsd.c
+++ b/coregrind/m_syswrap/syswrap-freebsd.c
@@ -7026,15 +7026,7 @@ PRE(sys_aio_readv)
          SET_STATUS_Failure( VKI_EBADF );
       } else {
          SizeT vec_count = (SizeT)iocb->aio_nbytes;
-#if defined(__clang__)
-#pragma clang diagnostic push
-         // yes, I know it is volatile
-#pragma clang diagnostic ignored "-Wcast-qual"
-#endif
-         struct vki_iovec* p_iovec  = (struct vki_iovec*)iocb->aio_buf;
-#if defined(__clang__)
-#pragma clang diagnostic pop
-#endif
+         struct vki_iovec* p_iovec  = (struct vki_iovec*)(uintptr_t)iocb->aio_buf;
          PRE_MEM_READ("aio_readv(iocb->aio_iov)", (Addr)p_iovec,  vec_count*sizeof(struct vki_iovec));
          if (ML_(safe_to_deref)(p_iovec, vec_count*sizeof(struct vki_iovec))) {
             for (SizeT i = 0U; i < vec_count; ++i) {
diff --git a/memcheck/mc_errors.c b/memcheck/mc_errors.c
index e9a8f3f02e..b8869733fd 100644
--- a/memcheck/mc_errors.c
+++ b/memcheck/mc_errors.c
@@ -1014,6 +1014,15 @@ void MC_(record_realloc_size_zero) ( ThreadId tid, Addr a )
 {
    MC_Error extra;
    tl_assert(VG_INVALID_THREADID != tid);
+   /*
+    * We can't fill the Block as in freemismatch above.
+    * That's because if realloc size zero frees we literally do that
+    * and transform the call into a free before bothering to get the
+    * old MC_Chunk.
+    *
+    * See VG_(maybe_record_error) for a description of how this gets
+    * filled on demand.
+    */
    extra.Err.ReallocSizeZero.ai.tag = Addr_Undescribed;
    VG_(maybe_record_error)( tid, Err_ReallocSizeZero, a, /*s*/NULL, &extra );
 }