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Mon, 09 Mar 2026 16:36:11 -0700 (PDT) From: Razvan Ghiorghe To: qemu-devel@nongnu.org Cc: helge.deller@gmx.de, laurent@vivier.eu, Razvan Ghiorghe Subject: [PATCH v3] linux-user: fix mremap with old_size=0 for shared mappings Date: Tue, 10 Mar 2026 01:30:32 +0200 Message-ID: <20260309233352.20760-1-razvanghiorghe16@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=2a00:1450:4864:20::332; envelope-from=razvanghiorghe16@gmail.com; helo=mail-wm1-x332.google.com X-Spam_score_int: -17 X-Spam_score: -1.8 X-Spam_bar: - X-Spam_report: (-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1773099415693158500 Content-Type: text/plain; charset="utf-8" When old_size is zero and old_address refers to a shareable mapping, mremap() should create a new mapping of the same pages according to the mremap(2) man page. The MREMAP_MAYMOVE flag must be specified in this case. Previously, QEMU's target_mremap() rejected this valid case with EFAULT during the initial validation, before checking for the special old_size =3D=3D 0 behaviour. This patch adds proper handling for old_size =3D=3D 0: - Validates that MREMAP_MAYMOVE flag is set (required by man spec) - Passes the call through to the host mremap() - Creates a new mapping without invalidating the original, with both being valid and sharing the same physical memory frames. - Ensures the new mapping address falls within the valid guest address region before returning it to the guest. Tested with the reproducer from the issue on qemu-riscv64, qemu-hppa, and qemu-aarch64. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3105 Signed-off-by: Razvan Ghiorghe Hi Heldge, Thank you for testing and detailed feedback! V3 fixes h2g_valid assertion failure on hppa by using mmap_find_vma() to ensure the new mapping stays within the valid guest adress region Tested on qemu-riscv64, qemu-hppa and qemu-aarch64. Reviewed-by: Helge Deller Tested-by: Helge Deller --- linux-user/mmap.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 52 insertions(+) diff --git a/linux-user/mmap.c b/linux-user/mmap.c index 07175e11d5..b41308840a 100644 --- a/linux-user/mmap.c +++ b/linux-user/mmap.c @@ -1117,6 +1117,58 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong= old_size, errno =3D EINVAL; return -1; } + + if (!old_size) { + if (!(flags & MREMAP_MAYMOVE)) { + errno =3D EINVAL; + return -1; + } + mmap_lock(); + if (flags & MREMAP_FIXED) { + host_addr =3D mremap(g2h_untagged(old_addr), old_size, new_siz= e, + flags, g2h_untagged(new_addr)); + } else { + /* + * We ensure that the new mapping stands in the + * region of guest mappable addresses. + */ + abi_ulong mmap_start; + + mmap_start =3D mmap_find_vma(0, new_size, TARGET_PAGE_SIZE); + + if (mmap_start =3D=3D -1) { + errno =3D ENOMEM; + mmap_unlock(); + return -1; + } + + host_addr =3D mremap(g2h_untagged(old_addr), old_size, new_siz= e, + flags | MREMAP_FIXED, g2h_untagged(mmap_start= )); + + new_addr =3D mmap_start; + } + =20 + if (host_addr =3D=3D MAP_FAILED) { + mmap_unlock(); + return -1; + } + + if (flags & MREMAP_FIXED) { + new_addr =3D h2g(host_addr); + } + + prot =3D page_get_flags(old_addr); + /* + * For old_size zero, there is nothing to clear at old_addr. + * Only set the flags for the new mapping. They both are valid. + */ + page_set_flags(new_addr, new_addr + new_size - 1, + prot | PAGE_VALID, PAGE_VALID); + shm_region_rm_complete(new_addr, new_addr + new_size - 1); + mmap_unlock(); + return new_addr; + } + if (!guest_range_valid_untagged(old_addr, old_size)) { errno =3D EFAULT; return -1; --=20 2.43.0