[Bug] Fix integer overflow in activation_kernels.cu pointer arithmetic

[dparikh79]

Summary

The per-token pointer offsets in three kernels in csrc/activation_kernels.cu were computed as blockIdx.x * 2 * d (or blockIdx.x * d) without promoting the index to a 64-bit type. blockIdx.x is unsigned int and d is int, so the product is evaluated in 32-bit arithmetic and overflows once it exceeds INT_MAX (about 2.15 billion).

When the overflow occurs the kernel reads / writes the wrong memory and the result is silently incorrect. For the reporter's case in #42860 (model: royokong/e5-v, d = 14336, seq_len = 7890, batch_size = 19), the token dimension is 149911, and token_idx * 2 * d = 4.29 billion, crossing the 32-bit boundary at row ~74955.

Fix

Adopt the const int64_t token_idx = blockIdx.x; pattern already used by swigluoai_and_mul_kernel further down in the same file (the only kernel here that was correct), and use token_idx instead of blockIdx.x in the affected pointer-offset expressions.

Affected kernels (all in csrc/activation_kernels.cu):

• act_and_mul_kernel (template entry point used by silu_and_mul, gelu_and_mul, fatrelu_and_mul, etc.)
• act_and_mul_kernel_with_param (used by activation kernels that take an extra scalar parameter)
• activation_kernel (the elementwise gelu_new, gelu_fast, gelu_quick template entry point)

The first occurrence gets a fuller explanatory comment; the others get a one-line back-reference so the rationale stays discoverable without duplication.

Test plan

• Static review: every blockIdx.x * <stride> pointer arithmetic in csrc/activation_kernels.cu is now backed by int64_t token_idx. swigluoai_and_mul_kernel was already correct and is unchanged.
• On-GPU repro from [Bug]: integer overflow in activation_kernels.cu #42860 (model royokong/e5-v, d=14336, seq_len=7890, batch_size=19): cannot run locally (no B300 / equivalent GPU); maintainers with the failing config can verify the corruption is gone. The static fix matches the pattern swigluoai_and_mul_kernel already used, so this is the expected behavior under the affected dimensions.

Out of scope for this PR (flagged so they are not lost)

The same class of 32-bit-multiply pointer-arithmetic pattern exists in:

• csrc/layernorm_kernels.cu lines 29, 69, 118, 136, 167-171 (blockIdx.x * hidden_size, blockIdx.x * input_stride, blockIdx.x * vec_hidden_size, etc.)
• csrc/fused_qknorm_rope_kernel.cu lines 150, 363 and csrc/fused_deepseek_v4_qnorm_rope_kv_insert_kernel.cu line 157 (warp-index calculations of the form blockIdx.x * warpsPerBlock + warpId)

Those follow the same fix shape but live in different code paths and have different exposure profiles. Happy to follow up in separate PRs if maintainers want them addressed.

Fixes #42860.

---

AI assistance disclosure

This PR was prepared with the assistance of an AI coding tool (Claude). The bug diagnosis, the fix, the static audit of all blockIdx.x * d sites in csrc/activation_kernels.cu, the int64_t-promotion pattern (matched to the existing swigluoai_and_mul_kernel), and the cross-file scan for adjacent occurrences were each reviewed by me, and I am responsible for the contents.

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[gemini-code-assist]

Code Review

This pull request fixes potential 32-bit integer overflow bugs in the act_and_mul_kernel, act_and_mul_kernel_with_param, and activation_kernel CUDA kernels by promoting blockIdx.x to int64_t before calculating memory offsets. This change ensures that pointer arithmetic remains valid for large hidden sizes. I have no feedback to provide.

[mgoin]

Thanks for the patch, could you please remove the excessive comment? One line at most imo @dparikh79

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @dparikh79.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[dparikh79]

#42663 moved this file two days before your review. Missed it at rebase time. Same close-and-reopen plan as for layernorm sibling #42863, with the fix re-applied to csrc/libtorch_stable/activation_kernels.cu. @mgoin if force-pushing in place reads better to you, can do that instead.

[mgoin]

@dparikh79 whatever works for you, the change would be good to get in!

[dparikh79]

Closing for #44026 (same fix at the post-#42663 path, comment-trim pre-applied). Thanks @mgoin.