| Message ID | 23bd38a4bf024d4a92a8a634ddf4d5689cd3a67e.1729802213.git.gustavoars@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | net: Avoid thousands of -Wflex-array-member-not-at-end warnings | expand |
> As this new struct will live in UAPI, to avoid breaking user-space code > that expects `struct sockaddr`, the `__kernel_sockaddr_legacy` macro is > introduced. This macro allows us to use either `struct sockaddr` or > `struct sockaddr_legacy` depending on the context in which the code is > used: kernel-space or user-space. Are there cases of userspace API structures where the flexiable array appears in the middle? I assume this new compiler flag is not only for use in the kernel? When it gets turned on in user space, will the kernel headers will again produce warnings? Should we be considering allowing user space to opt in to using sockaddr_legacy? Andrew
On Mon, 2024-10-28 at 21:38 +0100, Andrew Lunn wrote: > > As this new struct will live in UAPI, to avoid breaking user-space code > > that expects `struct sockaddr`, the `__kernel_sockaddr_legacy` macro is > > introduced. This macro allows us to use either `struct sockaddr` or > > `struct sockaddr_legacy` depending on the context in which the code is > > used: kernel-space or user-space. > > Are there cases of userspace API structures where the flexiable array > appears in the middle? Clearly, it's the case for all the three other patches in this series. > I assume this new compiler flag is not only for > use in the kernel? When it gets turned on in user space, will the > kernel headers will again produce warnings? Should we be considering > allowing user space to opt in to using sockaddr_legacy? For the userspace covered by patch 2 this will almost certainly never happen, and I suspect that might also be true for the others (arp and rtnetlink ioctls)? But it probably wouldn't be difficult either. johannes
On Mon, Oct 28, 2024 at 09:38:46PM +0100, Andrew Lunn wrote: > > As this new struct will live in UAPI, to avoid breaking user-space code > > that expects `struct sockaddr`, the `__kernel_sockaddr_legacy` macro is > > introduced. This macro allows us to use either `struct sockaddr` or > > `struct sockaddr_legacy` depending on the context in which the code is > > used: kernel-space or user-space. > > Are there cases of userspace API structures where the flexiable array > appears in the middle? I assume this new compiler flag is not only for > use in the kernel? When it gets turned on in user space, will the > kernel headers will again produce warnings? Should we be considering > allowing user space to opt in to using sockaddr_legacy? I expect that the userspace usage of -Wflex-array-member-not-at-end will be driven by the libc projects, as it'll need to happen there before it can be done anywhere else. We'll be able to coordinate with them at that time, but I'm not aware of any plans by any libc to use this flag yet.
On Thu, Oct 24, 2024 at 03:11:24PM -0600, Gustavo A. R. Silva wrote: > diff --git a/include/uapi/linux/socket.h b/include/uapi/linux/socket.h > index d3fcd3b5ec53..2e179706bec4 100644 > --- a/include/uapi/linux/socket.h > +++ b/include/uapi/linux/socket.h > @@ -35,4 +35,32 @@ struct __kernel_sockaddr_storage { > #define SOCK_TXREHASH_DISABLED 0 > #define SOCK_TXREHASH_ENABLED 1 > > +typedef __kernel_sa_family_t sa_family_t; > + > +/* > + * This is the legacy form of `struct sockaddr`. The original `struct sockaddr` > + * was modified in commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible > + * array in struct sockaddr") due to the fact that "One of the worst offenders > + * of "fake flexible arrays" is struct sockaddr". This means that the original > + * `char sa_data[14]` behaved as a flexible array at runtime, so a proper > + * flexible-array member was introduced. > + * > + * This caused several flexible-array-in-the-middle issues: > + * https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wflex-array-member-not-at-end > + * > + * `struct sockaddr_legacy` replaces `struct sockaddr` in all instances where > + * objects of this type do not appear at the end of composite structures. > + */ > +struct sockaddr_legacy { > + sa_family_t sa_family; /* address family, AF_xxx */ > + char sa_data[14]; /* 14 bytes of protocol address */ > +}; > + > +#ifdef __KERNEL__ > +# define __kernel_sockaddr_legacy sockaddr_legacy > +#else > +# define __kernel_sockaddr_legacy sockaddr > +#endif Yeah, this matches what I'd expect. Reviewed-by: Kees Cook <kees@kernel.org>
On Mon, Oct 28, 2024 at 09:47:08PM +0100, Johannes Berg wrote: > On Mon, 2024-10-28 at 21:38 +0100, Andrew Lunn wrote: > > > As this new struct will live in UAPI, to avoid breaking user-space code > > > that expects `struct sockaddr`, the `__kernel_sockaddr_legacy` macro is > > > introduced. This macro allows us to use either `struct sockaddr` or > > > `struct sockaddr_legacy` depending on the context in which the code is > > > used: kernel-space or user-space. > > > > Are there cases of userspace API structures where the flexiable array > > appears in the middle? > > Clearly, it's the case for all the three other patches in this series. The issue is that the kernel uses these structures, and the kernel's view of sockaddr is that it (correctly) has a flexible array. Userspace's view of sockaddr is the old struct (which comes from the libc, not the kernel) which ends with a fake flexible array. We need to correct the kernel's view of these structures to use the introduced legacy struct to avoid lying to the compiler about what's going on. :)
On Thu, 24 Oct 2024 15:11:24 -0600 Gustavo A. R. Silva wrote: > + * This is the legacy form of `struct sockaddr`. The original `struct sockaddr` > + * was modified in commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible > + * array in struct sockaddr") due to the fact that "One of the worst offenders > + * of "fake flexible arrays" is struct sockaddr". This means that the original > + * `char sa_data[14]` behaved as a flexible array at runtime, so a proper > + * flexible-array member was introduced. This isn't spelled out in the commit messages AFACT so let me ask.. Why aren't we reverting b5f0de6df6dce, then? Feels like the best solution would be to have a separate type with the flex array to clearly annotate users who treat it as such. Is that not going to work? My noob reading of b5f0de6df6dce is that it was a simpler workaround for the previous problem, avoided adding a new type (and the conversion churn). But now we are adding a type and another workaround on top. Sorry if I'm misunderstanding. No question that the struct is a mess, but I don't feel like this is helping the messiness...
On Thu, Oct 31, 2024 at 06:01:45PM -0700, Jakub Kicinski wrote: > On Thu, 24 Oct 2024 15:11:24 -0600 Gustavo A. R. Silva wrote: > > + * This is the legacy form of `struct sockaddr`. The original `struct sockaddr` > > + * was modified in commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible > > + * array in struct sockaddr") due to the fact that "One of the worst offenders > > + * of "fake flexible arrays" is struct sockaddr". This means that the original > > + * `char sa_data[14]` behaved as a flexible array at runtime, so a proper > > + * flexible-array member was introduced. > > This isn't spelled out in the commit messages AFACT so let me ask.. > Why aren't we reverting b5f0de6df6dce, then? > Feels like the best solution would be to have a separate type with > the flex array to clearly annotate users who treat it as such. > Is that not going to work? > > My noob reading of b5f0de6df6dce is that it was a simpler workaround > for the previous problem, avoided adding a new type (and the conversion > churn). But now we are adding a type and another workaround on top. > Sorry if I'm misunderstanding. No question that the struct is a mess, > but I don't feel like this is helping the messiness... This is a pretty reasonable position. I think sockaddr turns out to be a really difficult problem, given its exposure to UAPI. Linux has been trying to deal with it for a while (e.g. sockaddr_storage), and perhaps b5f0de6df6dce wasn't the right solution. (It looked correct since it looks like what we've done in many other tricky places, but perhaps sockaddr should be dealt with differently yet.) So, the rationale with b5f0de6df6dce was to change things as minimally as possible. The goal is to stop lying to the compiler about object sizes, with the big lie being "this object ends with a 14 byte array". I think this results in two primary goals: 1- make sockaddr-like objects unambiguously sized 2- do not break userspace For 2, we cannot change the existing (and externally defined) struct sockaddr. It will stay the historical horrible thing that it is. The specific UAPI sockaddr-related things we have that are _fixed_ sizes are all fine. These can be seen with: git grep '^struct sockaddr_' -- include/uapi/ For 1, we cannot use (the externally defined) sockaddr as-is since we run into size problems. (Basically anything casting from sockaddr, anything copying out of sockaddr, etc.) The storage problem for sockaddr has been attempted to be addressed with the internally defined sockaddr_storage, but this doesn't capture the "I don't know how big this object is yet" issues associated with taking a buffer of arbitrary size from userspace that is initially defined as sockaddr. If we internally add struct sockaddr_unknown, or _bytes, or _unspec, or _flex, or _array, or _raw, we can use that everywhere in the kernel, but it is going to cause a lot of churn. This is why b5f0de6df6dce happened, and why we went the direction of thinking sockaddr_legacy would be workable: the kernel's view of sockaddr from userspace (sockaddr_legacy) would be the only change needed. Now, if we want to get to a place with the least ambiguity, we need to abolish sockaddr from the kernel internally, and I think that might take a while. I only think so because of what I went through just for doing a portion of NFS in cf0d7e7f4520 ("NFS: Avoid memcpy() run-time warning for struct sockaddr overflows"). But maybe it won't be as bad all that since we already do a lot of "what is the family then cast to a fixed-size struct and carry on" stuff. But the passing things between layers (usually "up" from a family into the networking core) still depends on using sockaddr, and all of those APIs would need to switch to sockaddr_unknown both in the core and in all families. Maybe there is some Coccinelle magic that could be worked. I think it's worth spending the time to try and figure out the size of the effort needed to make that change. -Kees
diff --git a/include/uapi/linux/socket.h b/include/uapi/linux/socket.h index d3fcd3b5ec53..2e179706bec4 100644 --- a/include/uapi/linux/socket.h +++ b/include/uapi/linux/socket.h @@ -35,4 +35,32 @@ struct __kernel_sockaddr_storage { #define SOCK_TXREHASH_DISABLED 0 #define SOCK_TXREHASH_ENABLED 1 +typedef __kernel_sa_family_t sa_family_t; + +/* + * This is the legacy form of `struct sockaddr`. The original `struct sockaddr` + * was modified in commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible + * array in struct sockaddr") due to the fact that "One of the worst offenders + * of "fake flexible arrays" is struct sockaddr". This means that the original + * `char sa_data[14]` behaved as a flexible array at runtime, so a proper + * flexible-array member was introduced. + * + * This caused several flexible-array-in-the-middle issues: + * https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wflex-array-member-not-at-end + * + * `struct sockaddr_legacy` replaces `struct sockaddr` in all instances where + * objects of this type do not appear at the end of composite structures. + */ +struct sockaddr_legacy { + sa_family_t sa_family; /* address family, AF_xxx */ + char sa_data[14]; /* 14 bytes of protocol address */ +}; + +#ifdef __KERNEL__ +# define __kernel_sockaddr_legacy sockaddr_legacy +#else +# define __kernel_sockaddr_legacy sockaddr +#endif + + #endif /* _UAPI_LINUX_SOCKET_H */
We are currently working on enabling the -Wflex-array-member-not-at-end compiler option. This option has helped us detect several objects of the type `struct sockaddr` that appear in the middle of composite structures like `struct rtentry`, `struct compat_rtentry`, and others: include/uapi/linux/wireless.h:751:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/wireless.h:776:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/wireless.h:833:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/wireless.h:857:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/wireless.h:864:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/route.h:33:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/route.h:34:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/route.h:35:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/if_arp.h:118:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/if_arp.h:119:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/if_arp.h:121:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/if_arp.h:126:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/uapi/linux/if_arp.h:127:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/net/compat.h:34:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] include/net/compat.h:35:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] In order to fix the warnings above, we introduce `struct sockaddr_legacy`. The intention is to use it to replace the type of several struct members in the middle of composite structures, currently of type `struct sockaddr`. These middle struct members are currently causing thousands of warnings because `struct sockaddr` contains a flexible-array member, introduced by commit b5f0de6df6dce ("net: dev: Convert sa_data to flexible array in struct sockaddr"). The new `struct sockaddr_legacy` doesn't include a flexible-array member, making it suitable for use as the type of middle members in composite structs that don't really require the flexible-array member in `struct sockaddr`, thus avoiding -Wflex-array-member-not-at-end warnings. As this new struct will live in UAPI, to avoid breaking user-space code that expects `struct sockaddr`, the `__kernel_sockaddr_legacy` macro is introduced. This macro allows us to use either `struct sockaddr` or `struct sockaddr_legacy` depending on the context in which the code is used: kernel-space or user-space. Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org> --- include/uapi/linux/socket.h | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+)