Re: [PATCH bpf v2 2/2] bpf: Avoid faultable build ID reads under mm locks

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 4/9/26 5:20 AM, Ihor Solodrai wrote:
> 
> 
> On 4/8/26 6:06 PM, Ihor Solodrai wrote:
>> Sleepable build ID parsing can block in __kernel_read() [1], so the
>> stackmap sleepable path must not call it while holding mmap_lock or a
>> per-VMA read lock.
>>
>> The issue and the fix are conceptually similar to a recent procfs
>> patch [2].
>>
>> Resolve each covered VMA with a stable read-side reference, preferring
>> lock_vma_under_rcu() and falling back to mmap_read_lock() only long
>> enough to acquire the VMA read lock. Take a reference to the backing
>> file, drop the VMA lock, and then parse the build ID through
>> (sleepable) build_id_parse_file().
>>
>> [1]: https://lore.kernel.org/all/20251218005818.614819-1-shakeel.butt@linux.dev/
>> [2]: https://lore.kernel.org/all/20260128183232.2854138-1-andrii@kernel.org/
>>
>> Fixes: 777a8560fd29 ("lib/buildid: use __kernel_read() for sleepable context")
>> Assisted-by: Codex:gpt-5.4
>> Suggested-by: Puranjay Mohan <puranjay@kernel.org>
>> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
>> ---
>>   kernel/bpf/stackmap.c | 139 ++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 139 insertions(+)
>>
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 4ef0fd06cea5..de3d89e20a1e 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -9,6 +9,7 @@
>>   #include <linux/perf_event.h>
>>   #include <linux/btf_ids.h>
>>   #include <linux/buildid.h>
>> +#include <linux/mmap_lock.h>
>>   #include "percpu_freelist.h"
>>   #include "mmap_unlock_work.h"
>>   
>> @@ -158,6 +159,139 @@ static inline void stack_map_build_id_set_ip(struct bpf_stack_build_id *id)
>>   	memset(id->build_id, 0, BUILD_ID_SIZE_MAX);
>>   }
>>   
>> +enum stack_map_vma_lock_state {
>> +	STACK_MAP_LOCKED_NONE = 0,
>> +	STACK_MAP_LOCKED_VMA,
>> +	STACK_MAP_LOCKED_MMAP,
>> +};
>> +
>> +struct stack_map_vma_lock {
>> +	enum stack_map_vma_lock_state state;
>> +	struct vm_area_struct *vma;
>> +	struct mm_struct *mm;
>> +};
>> +
>> +static struct vm_area_struct *stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip)
>> +{
>> +	struct mm_struct *mm = lock->mm;
>> +	struct vm_area_struct *vma;
>> +
>> +	if (WARN_ON_ONCE(!mm))
>> +		return NULL;
>> +
>> +	vma = lock_vma_under_rcu(mm, ip);
>> +	if (vma)
>> +		goto vma_locked;
>> +
>> +	if (!mmap_read_trylock(mm))
>> +		return NULL;
>> +
>> +	vma = vma_lookup(mm, ip);
>> +	if (!vma) {
>> +		mmap_read_unlock(mm);
>> +		return NULL;
>> +	}
>> +
>> +#ifdef CONFIG_PER_VMA_LOCK
> 
> Puranjay and others,
> 
> I tried to test this patch with CONFIG_PER_VMA_LOCK=n, and it turns
> out it's not easy to turn off.
> 
>      config PER_VMA_LOCK
>      	def_bool y
>      	depends on ARCH_SUPPORTS_PER_VMA_LOCK && MMU && SMP
> 
> AFAIU this definition means that PER_VMA_LOCK is automatically true if
> it's dependencies are satisfied. The only practical way to do it
> appears to be disabling SMP, which is probably very unusual.
> 
> So I am thinking, maybe instead of having to manage both mmap and vma
> lock in stackmap.c, we can assume CONFIG_PER_VMA_LOCK=y, and have
> 
>      #ifndef CONFIG_PER_VMA_LOCK
>      return -ENOTSUPP
>      #endif
> 
> somewhere in the stack_map_get_build_id_offset() callstack?
> Say in __bpf_get_stackid()?
> 
> Thoughts?
>

I looks like you are right, it should be safe to drop 
CONFIG_PER_VMA_LOCK=n case, I understand all platforms that care about 
BPF going to have it enabled.

>> +	if (!vma_start_read_locked(vma)) {
>> +		mmap_read_unlock(mm);
>> +		return NULL;
>> +	}
>> +	mmap_read_unlock(mm);
>> +#else
>> +	lock->state = STACK_MAP_LOCKED_MMAP;
>> +	lock->vma = vma;
>> +	return vma;
>> +#endif
>> +
>> +vma_locked:
>> +	lock->state = STACK_MAP_LOCKED_VMA;
>> +	lock->vma = vma;
>> +	return vma;
>> +}
>> +
>> +static void stack_map_unlock_vma(struct stack_map_vma_lock *lock)
>> +{
>> +	struct vm_area_struct *vma = lock->vma;
>> +	struct mm_struct *mm = lock->mm;
>> +
>> +	switch (lock->state) {
>> +	case STACK_MAP_LOCKED_VMA:
>> +		if (WARN_ON_ONCE(!vma))
>> +			break;
>> +		vma_end_read(vma);
>> +		break;
>> +	case STACK_MAP_LOCKED_MMAP:
>> +		if (WARN_ON_ONCE(!mm))
>> +			break;
>> +		mmap_read_unlock(mm);
>> +		break;
>> +	default:
>> +		break;
>> +	}
>> +
>> +	lock->state = STACK_MAP_LOCKED_NONE;
>> +	lock->vma = NULL;
>> +}
>> +
>> +static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *id_offs,
>> +						    u32 trace_nr)
>> +{
>> +	struct mm_struct *mm = current->mm;
>> +	struct stack_map_vma_lock lock = {
>> +		.state = STACK_MAP_LOCKED_NONE,
>> +		.vma = NULL,
>> +		.mm = mm,
>> +	};
>> +	struct file *file, *prev_file = NULL;
>> +	unsigned long vm_pgoff, vm_start;
>> +	struct vm_area_struct *vma;
>> +	const char *prev_build_id;
>> +	u64 ip;
>> +
>> +	for (u32 i = 0; i < trace_nr; i++) {
>> +		ip = READ_ONCE(id_offs[i].ip);
>> +		vma = stack_map_lock_vma(&lock, ip);
>> +		if (!vma || !vma->vm_file) {
>> +			stack_map_build_id_set_ip(&id_offs[i]);
>> +			stack_map_unlock_vma(&lock);
>> +			continue;
>> +		}
>> +
>> +		file = vma->vm_file;
>> +		vm_pgoff = vma->vm_pgoff;
>> +		vm_start = vma->vm_start;
>> +
>> +		if (file == prev_file) {
>> +			memcpy(id_offs[i].build_id, prev_build_id, BUILD_ID_SIZE_MAX);
>> +			stack_map_unlock_vma(&lock);
>> +			goto build_id_valid;
>> +		}
>> +
>> +		file = get_file(file);
>> +		stack_map_unlock_vma(&lock);
>> +
>> +		/* build_id_parse_file() may block on filesystem reads */
>> +		if (build_id_parse_file(file, id_offs[i].build_id, NULL)) {
>> +			stack_map_build_id_set_ip(&id_offs[i]);
>> +			fput(file);
>> +			continue;
>> +		}
>> +
>> +		if (prev_file)
>> +			fput(prev_file);
>> +		prev_file = file;
>> +		prev_build_id = id_offs[i].build_id;
>> +
>> +build_id_valid:
>> +		id_offs[i].offset = (vm_pgoff << PAGE_SHIFT) + ip - vm_start;
>> +		id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
>> +	}
>> +
>> +	if (prev_file)
>> +		fput(prev_file);
>> +}
>> +
>>   /*
>>    * Expects all id_offs[i].ip values to be set to correct initial IPs.
>>    * They will be subsequently:
>> @@ -178,6 +312,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>>   	const char *prev_build_id;
>>   	int i;
>>   
>> +	if (may_fault && has_user_ctx) {
>> +		stack_map_get_build_id_offset_sleepable(id_offs, trace_nr);
>> +		return;
>> +	}
>> +
>>   	/* If the irq_work is in use, fall back to report ips. Same
>>   	 * fallback is used for kernel stack (!user) on a stackmap with
>>   	 * build_id.
> 
>