Backup failed due to insufficient S3 permissions under the Web Identity role.

[3pointer]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1.	Configure a Pod to assume an IAM role via Web Identity (IRSA) that grants S3 access permissions.
2.	Specify a custom S3 endpoint (e.g., a FIPS endpoint) in the backup configuration.
3.	Execute a full backup operation.

2. What did you expect to see? (Required)

Backup success.

3. What did you see instead (Required)

Backup failed.

4. What is your TiDB version? (Required)

master