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From: Thorsten Blum To: Lorenzo Stoakes Cc: Andrew Morton , David Hildenbrand , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Andy Lutomirski , Thomas Gleixner , Vincenzo Frascino , Kees Cook , Andy Shevchenko , Yury Norov , David Laight , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/2] string: use offset_in_page() in sized_strscpy() Message-ID: References: <20260521090655.160282-4-thorsten.blum@linux.dev> <20260521090655.160282-6-thorsten.blum@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 5995D100005 X-Stat-Signature: ofw661qebgy1iedozpfxjfuc1tfej915 X-Rspamd-Server: rspam06 X-HE-Tag: 1779371253-675126 X-HE-Meta: U2FsdGVkX19QLSYbsUjPRE4qQb/69mCEMj5FleoRycQH65Woo6gvoEb4cp2qtiuJqUjLFuFyDG8ZNwEQ0/83z70LhkbjxExhFH0PlvmVzcJLinpslgOeEQEp1wUFLgp8ZtzVo7qbABpjG793gcUi0PUjVrJk20JYOVhekm4+wJuZVOeWBzX532XWhJFWLDdBMwrvseq5A8dV3unV4HOA0HYQ09teRjO9ideMia5f7InDB6iCJvIS4M3f3oG59s/kNEKPXjQce8d4otXSpLJRSh/hyatXezXZvXLBNWvVTx59GDAImOe5FmvVdYAqtbG685pIX1rzJJI9KTmmLozWMcaz7GIL8c57on72Rq3Y1nvGuzSdhMcFiSVhYqlKaeuzHyGusSKPglONX2IBZZack8eNllqjqLeImXmcNHeAgYLePTn72RIX7i6qI5gIxy+ZAcvk5xk4OlM64pW5zkq0H2zHlml72iTWy6oVN472HZ3FQRQCH0+CETetsND4aJAE9j+0VVSqL758W2uhiD98GYgT5b3FcWgXxsAGtxkPPKGCopOYWqQMRdmuJ/2CsM6MRwreFHAZHNzQmv0OIMQCoyubFrjfIOWicsheFis9tUSpA03frdc48PL7AhBFZJUgdzE3zQReL7mzgmJd+CZz1S/EJOjAyyNub7EuYzS3TwwOt52NvPc2a7bgREIBmJ5buMkUv5j3DYRPk71bbM8wqfWsxY3N1opIIXnTQjh3PiGXtjDj3XXbhNEI9+n9AZrDIad0JTHQy9QcTtjDUh22KBfx+WdotydeIKE6GwjKK8VWwkrE9kD+fINMynVkKzNwQF19FEiPQgptGL7lMUonzyoC85Kho4bW75PesAdrnXj+oSuD+ZscvqNLZYln+F+jXoaDjBZUGWJ2eLeD1BcIOLENwZVIkPAjSqZX8UlOuObY0lYjj9Bb/9I4Bw3YKjkcZ8XAEUP5BbKv8M07l5g 8ZIHiVDb yftytbkdIBAP8c8RTu3OpkN+e8Myz62wt8gjnwT5NseX8jA3WHP6NBQNXEHgOuUJKhS74gk1IQGdc7Vb9n8sR6hqfkdfnJbKTZSx6d3iCpvd0k1cIoLz1xG1gn6AoIFVXC6wMUJpgEben8m519oEt6NEhGWDt0Gp6QH3gFwKTKK9UrLqX75Rhnecc5zBV8J0x0jm1qz0/wtLafIVgpVSGwYMtKo2G5J87gxiLdDt0dcQ+AtAIk8hsdcv+BeGDNyaVMFgrQCwmobSlSio1N5OF2qh8vhAjcYVMZrUsOi3frpiH62dx8+MMYIWJt4oJf+DHf4ZWXvRaU1kz/VhUKqDLhDrWV3x6m3FRs6AZMZ+uIW+pmIlrLUrvU7+9Mu0AjiP/FK6Ovai8JNcM21B3Xz7/Qby749TtRr3U5cTS+XElH+pZWdYDIzgnr+mwvA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, May 21, 2026 at 12:31:25PM +0100, Lorenzo Stoakes wrote: > On Thu, May 21, 2026 at 11:06:58AM +0200, Thorsten Blum wrote: > > Replace the open-coded implementation with offset_in_page() to simplify > > sized_strscpy(). > > > > Signed-off-by: Thorsten Blum > > I feel this patch actually makes sized_strscpy() more difficult to understand > unfortunately, so not really in favour of us taking it. > > > --- > > lib/string.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/lib/string.c b/lib/string.c > > index 1f9297e9776a..7c72adc7377c 100644 > > --- a/lib/string.c > > +++ b/lib/string.c > > @@ -25,6 +25,7 @@ > > #include > > #include > > #include > > +#include > > > > #include > > #include > > @@ -127,7 +128,7 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) > > * since we don't know if the next page is mapped. > > */ > > if ((long)src & (sizeof(long) - 1)) > > - max = min(PAGE_SIZE - ((long)src & (PAGE_SIZE - 1)), max); > > + max = min(PAGE_SIZE - offset_in_page(src), max); > > This isn't strictly the same code, offset_in_page() is: > > #define offset_in_page(p) ((unsigned long)(p) & ~PAGE_MASK) > > So this is now, effectively: > > - max = min(PAGE_SIZE - ((long)src & (PAGE_SIZE - 1)), max); > + max = min(PAGE_SIZE - ((unsigned long)src & (PAGE_SIZE -1)), max); > > So there could be some issues here with type conversions at least in theory. I think this should have used unsigned long from the start, as long seems a bit odd for a pointer, and PAGE_SIZE - offset_in_page() is a common kernel idiom for this exact calculation. Andrew also commented [1] on this line recently. > But in any case you're now making the logic inconsistent (the next line uses > bitwise operations directly). Agreed, maybe there are other helpers we can use here too, but that should probably be a follow-up patch. > So I'd rather we didn't make this change. FWIW, using offset_in_page() in lib/string.c (as suggested by Andy [2]) was the initial motivation for moving it out of mm.h, so I'd prefer to keep the use-site in this series if possible. Thanks, Thorsten [1] https://lore.kernel.org/lkml/20260519123740.458d905f958f39d41cb130fd@linux-foundation.org/ [2] https://lore.kernel.org/lkml/CAHp75VfQNkqEYsO4Uup0c-uiYuVyAWit=tmCz2BsYLp-sjXsZw@mail.gmail.com/