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Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pedro Falcato , linux-fsdevel@vger.kernel.org Subject: Re: [RFC PATCH 3/6] mm: nommu: fix an issue on map request to /dev/zero Message-ID: <2026081355-remold-sterility-84f9@gregkh> References: <20260813063401.1786548-1-thehajime@gmail.com> <20260813063401.1786548-4-thehajime@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260813063401.1786548-4-thehajime@gmail.com> X-Rspam-User: X-Rspamd-Queue-Id: B7E5AA0004 X-Rspamd-Server: rspam07 X-Stat-Signature: 16q6k4i4uy8oumfxmka9uyru97damqx4 X-HE-Tag: 1786623989-57627 X-HE-Meta: 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 sVkd/1Ef 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 13, 2026 at 03:33:58PM +0900, Hajime Tazaki wrote: > Upon a private file mapping request to /dev/zero, it calls > kernel_read() in do_mmap_private(), getting a failure with the message > like: "kernel reads not supported for file /dev/zero", which is because > zero_fops defined in drivers/char/mem.c has both .read and .read_iter > definitions. Do you actually use a no-mmu system? > Even fixing this issue, the map request to /dev/zero works fine without > errors but the allocated vma isn't marked with anonymous because > mmap_zero_prepare() isn't called under nommu platform, resulting > vma_desc_set_anonymous() isn't called either. > > This commit fixes those issues by: > 1) use vfs_iter_read() instead to avoid failure at kernel_read() > 2) calls .mmap_prepare on private mapping in do_mmap() so that required > preparations are done even in private mapping. > > Cc: Arnd Bergmann > Cc: Greg Kroah-Hartman > Cc: "Matthew Wilcox (Oracle)" > Cc: Jan Kara > Cc: Andrew Morton > Cc: "Liam R. Howlett" > Cc: Lorenzo Stoakes > Cc: Vlastimil Babka > Cc: Jann Horn > Cc: Pedro Falcato > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-mm@kvack.org (open list:PAGE CACHE) > Fixes: 4d03e3cc5982 ("fs: don't allow kernel reads and writes without iter ops") Given the age of this issue, I don't think anyone uses no-mmu systems anymore :( > Assisted-by: cubic.dev:unspecified > Signed-off-by: Hajime Tazaki > --- > drivers/char/mem.c | 5 ++- > mm/filemap.c | 6 ++-- > mm/nommu.c | 84 ++++++++++++++++++++++++++++++++++++++++++++-- > 3 files changed, 87 insertions(+), 8 deletions(-) > > diff --git a/drivers/char/mem.c b/drivers/char/mem.c > index 63253d1de5d7..dba24d0a7b33 100644 > --- a/drivers/char/mem.c > +++ b/drivers/char/mem.c > @@ -500,11 +500,10 @@ static ssize_t read_zero(struct file *file, char __user *buf, > > static int mmap_zero_prepare(struct vm_area_desc *desc) > { > -#ifndef CONFIG_MMU > - return -ENOSYS; > -#endif > +#ifdef CONFIG_MMU > if (vma_desc_test(desc, VMA_SHARED_BIT)) > return shmem_zero_setup_desc(desc); > +#endif > > /* > * This is a highly unique situation where we mark a MAP_PRIVATE mapping > diff --git a/mm/filemap.c b/mm/filemap.c > index d721986d5f46..cf02faad86aa 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -4077,7 +4077,7 @@ int generic_file_mmap(struct file *file, struct vm_area_struct *vma) > } > int generic_file_mmap_prepare(struct vm_area_desc *desc) > { > - return -ENOSYS; > + return 0; > } > int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma) > { > @@ -4085,7 +4085,9 @@ int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma) > } > int generic_file_readonly_mmap_prepare(struct vm_area_desc *desc) > { > - return -ENOSYS; > + if (is_shared_maywrite(&desc->vma_flags)) > + return -EINVAL; > + return generic_file_mmap_prepare(desc); > } > #endif /* CONFIG_MMU */ > > diff --git a/mm/nommu.c b/mm/nommu.c > index e40990e15831..a29a53c1c80a 100644 > --- a/mm/nommu.c > +++ b/mm/nommu.c > @@ -37,6 +37,7 @@ > > #include > #include > +#include > #include > #include > #include > @@ -856,6 +857,22 @@ static int validate_mmap_request(struct file *file, > return 0; > } > > +static int is_file_anonymous(struct file *file) > +{ > + if (!file) > + return 1; > + > + if (file->f_path.dentry && file->f_path.dentry->d_inode) { > + struct inode *inode = file->f_path.dentry->d_inode; > + /* if the device is /dev/zero */ > + if (S_ISCHR(inode->i_mode) && > + imajor(inode) == MEM_MAJOR && iminor(inode) == 5) > + return 1; > + } > + > + return 0; > +} > + > /* > * we've determined that we can make the mapping, now translate what we > * now know into VMA flags > @@ -869,7 +886,11 @@ static vm_flags_t determine_vm_flags(struct file *file, > > vm_flags = calc_vm_prot_bits(prot, 0) | calc_vm_flag_bits(file, flags); > > - if (!file) { > + /* private and file mapping will be marked anonymous later (do_mmap_private()). > + * and /dev/zero is marked by them at .mmap_prepare, > + * which should be _before_ this point. > + */ Wrong coding style for the comment, which is very typical of LLM-generated stuff. Always rewrite the output of these tools to actually be sane. thanks, greg k-h