regset: use kvzalloc() for regset_get_alloc()

While browsing through ChromeOS crash reports, I found one with an
allocation failure that looked like this:

chrome: page allocation failure: order:7

regset: use kvzalloc() for regset_get_alloc()

While browsing through ChromeOS crash reports, I found one with an
allocation failure that looked like this:

chrome: page allocation failure: order:7,
mode:0x40dc0(GFP_KERNEL|__GFP_COMP|__GFP_ZERO),
nodemask=(null),cpuset=urgent,mems_allowed=0
CPU: 7 PID: 3295 Comm: chrome Not tainted
5.15.133-20574-g8044615ac35c #1 (HASH:1162 1)
Hardware name: Google Lazor (rev3 - 8) with KB Backlight (DT)
Call trace:
...
warn_alloc+0x104/0x174
__alloc_pages+0x5f0/0x6e4
kmalloc_order+0x44/0x98
kmalloc_order_trace+0x34/0x124
__kmalloc+0x228/0x36c
__regset_get+0x68/0xcc
regset_get_alloc+0x1c/0x28
elf_core_dump+0x3d8/0xd8c
do_coredump+0xeb8/0x1378
get_signal+0x14c/0x804
...

An order 7 allocation is (1 << 7) contiguous pages, or 512K. It's not
a surprise that this allocation failed on a system that's been running
for a while.

More digging showed that it was fairly easy to see the order 7
allocation by just sending a SIGQUIT to chrome (or other processes) to
generate a core dump. The actual amount being allocated was 279,584
bytes and it was for "core_note_type" NT_ARM_SVE.

There was quite a bit of discussion [1] on the mailing lists in
response to my v1 patch attempting to switch to vmalloc. The overall
conclusion was that we could likely reduce the 279,584 byte allocation
by quite a bit and Mark Brown has sent a patch to that effect [2].
However even with the 279,584 byte allocation gone there are still
65,552 byte allocations. These are just barely more than the 65,536
bytes and thus would require an order 5 allocation.

An order 5 allocation is still something to avoid unless necessary and
nothing needs the memory here to be contiguous. Change the allocation
to kvzalloc() which should still be efficient for small allocations
but doesn't force the memory subsystem to work hard (and maybe fail)
at getting a large contiguous chunk.

[1] https://lore.kernel.org/r/20240201171159.1.Id9ad163b60d21c9e56c2d686b0cc9083a8ba7924@changeid
[2] https://lore.kernel.org/r/20240203-arm64-sve-ptrace-regset-size-v1-1-2c3ba1386b9e@kernel.org

Link: https://lkml.kernel.org/r/20240205092626.v2.1.Id9ad163b60d21c9e56c2d686b0cc9083a8ba7924@changeid
Signed-off-by: Douglas Anderson <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Dave Martin <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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regset: kill ->get()

no instances left

Signed-off-by: Al Viro <[email protected]>

regset: new method and helpers for it

->regset_get() takes task+regset+buffer, returns the amount of free space
left in the buffer on success and -E... on error.

buffer is represented as struct mem

regset: new method and helpers for it

->regset_get() takes task+regset+buffer, returns the amount of free space
left in the buffer on success and -E... on error.

buffer is represented as struct membuf - a pair of (kernel) pointer
and amount of space left

Primitives for writing to such:
* membuf_write(buf, data, size)
* membuf_zero(buf, size)
* membuf_store(buf, value)

These are implemented as inlines (in case of membuf_store - a macro).
All writes are sequential; they become no-ops when there's no space
left. Return value of all primitives is the amount of space left
after the operation, so they can be used as return values of ->regset_get().

Example of use:

// stores pt_regs of task + 64 bytes worth of zeroes + 32bit PID of task
int foo_get(struct task_struct *task, const struct regset *regset,
struct membuf to)
{
membuf_write(&to, task_pt_regs(task), sizeof(struct pt_regs));
membuf_zero(&to, 64);
return membuf_store(&to, (u32)task_tgid_vnr(task));
}

regset_get()/regset_get_alloc() taught to use that thing if present.
By the end of the series all users of ->get() will be converted;
then ->get() and ->get_size() can go.

Note that unlike ->get() this thing always starts at offset 0 and,
since it only writes to kernel buffer, can't fail on copyout.
It can, of course, fail for other reasons, but those tend to
be less numerous.

The caller guarantees that the buffer size won't be bigger than
regset->n * regset->size. That simplifies life for quite a few
instances.

Signed-off-by: Al Viro <[email protected]>

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copy_regset_to_user(): do all copyout at once.

Turn copy_regset_to_user() into regset_get_alloc() + copy_to_user().
Now all ->get() calls have a kernel buffer as destination.

Note that we'd already

copy_regset_to_user(): do all copyout at once.

Turn copy_regset_to_user() into regset_get_alloc() + copy_to_user().
Now all ->get() calls have a kernel buffer as destination.

Note that we'd already eliminated the callers of copy_regset_to_user()
with non-zero offset; now that argument is simply unused.

Uninlined, while we are at it.

Signed-off-by: Al Viro <[email protected]>

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introduction of regset ->get() wrappers, switching ELF coredumps to those

Two new helpers: given a process and regset, dump into a buffer.
regset_get() takes a buffer and size, regset_get_alloc() ta

introduction of regset ->get() wrappers, switching ELF coredumps to those

Two new helpers: given a process and regset, dump into a buffer.
regset_get() takes a buffer and size, regset_get_alloc() takes size
and allocates a buffer.

Return value in both cases is the amount of data actually dumped in
case of success or -E... on error.

In both cases the size is capped by regset->n * regset->size, so
->get() is called with offset 0 and size no more than what regset
expects.

binfmt_elf.c callers of ->get() are switched to using those; the other
caller (copy_regset_to_user()) will need some preparations to switch.

Signed-off-by: Al Viro <[email protected]>

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